Obd2 diagnostics

S
ome scan tools call it the
global OBD II mode, while
others describe it as the
OBD II generic mode. The
OBD II generic mode allows
a technician to attach his
scan tool to an OBD II-compliant vehi-
cle and begin collecting data without
entering any VIN information into the
scan tool. You may need to specifically
select “OBD II Generic” from the scan
tool menu. Some scan tools may need a
software module or personality key be-
fore they’ll work in generic OBD II test
mode.
The original list of generic data pa-
rameters mandated by OBD II and de-
scribed in SAE J1979 was short and de-
signed to provide critical system data
only. The useful types of data we can re-
trieve from OBD II generic include
short-term and long-term fuel trim val-
ues, oxygen sensor voltages, engine and
intake air temperatures, MAF or MAP
values, rpm, calculated load, spark tim-
ing and diagnostic trouble code (DTC)
count. Freeze frame data and readiness
status also are available in OBD II
generic mode. A generic scan tool also
should be able to erase trouble codes
and freeze frame data when command-
ed to do so.
Data coming to the scan tool through
the mandated OBD II generic interface
may not arrive as fast as data sent over
one of the dedicated data link connec-
tor (DLC) terminals. The vehicle man-
ufacturer has the option of using a
faster data transfer speed on other DLC
pins. Data on the generic interface also
may not be as complete as the informa-
tion you’ll get on many manufacturer-
52 September 2007
OBDII
GENERIC
PID
DIAGNOSISBY KARL SEYFERT
A wealth of diagnostic information is
available on late-model OBD II-compliant
vehicles, even when ‘enhanced’ or
‘manufacturer-specific’ PIDs are not
accessible. It doesn’t take much to use
this information to its best advantage.
Photo:KarlSeyfert

specific or enhanced interfaces. For ex-
ample, you may see an engine coolant
temperature (ECT) value in degrees on
the OBD II generic parameter identi-
fication (PID) list. A manufacturer-
specific data list may display ECT status
in Fahrenheit or Celsius and add a sep-
arate PID for the ECT signal voltage.
In spite of these and other limitations,
OBD II generic mode still contains
many of the trouble codes, freeze frame
data and basic datastream information
needed to solve many emissions-related
issues.
There are nine modes of operation
described in the original J1979 OBD II
standard. They are:
Mode 1: Show current data
Mode 2: Show freeze frame data
Mode 3: Show stored trouble codes
Mode 4: Clear trouble codes and stored
values
Mode 5: Test results, oxygen sensors
Mode 6: Test results, noncontinuously
monitored
Mode 7: Show pending trouble codes
Mode 8: Special control mode
Mode 9: Request vehicle information
Modes 1 and 2 are basically identical.
Mode 1 provides current information,
Mode 2 a snapshot of the same data
taken at the point when the last diag-
nostic trouble code was set. The excep-
tions are PID 01, which is available only
in Mode 1, and PID 02, available only
in Mode 2. If Mode 2 PID 02 returns
zero, then there’s no snapshot and all
other Mode 2 data is meaningless. Vehi-
cle manufacturers are not required to
support all modes. Each manufacturer
may define additional modes above
Mode 9 for other information.
Most vehicles from the J1979 era sup-
ported 13 to 20 parameters. The recent
phase-in of new parameters will make
OBD II generic data even more valu-
able. The California Air Resources
Board (CARB) revisions to OBD II
CAN-equipped vehicles have increased
the number of potential generic param-
eters to more than a hundred. Not all
vehicles will support all PIDs, and there
are many manufacturer-defined PIDs
that are not included in the OBD II
standard. Even so, the quality and
quantity of data have increased signifi-
cantly. For more information on the new
PIDs that were added to 2004 and later
CAN-equipped vehicles, refer to Bob
Pattengale’s article “Interpreting Gener-
ic Scan Data” in the March 2005 issue of
MOTOR. A PDF copy of the article can
be downloaded at www.motor.com.
Establish a Baseline
If you’re repairing a vehicle that has
stored one or more DTCs, make sure
you collect the freeze frame data before
erasing the stored codes. This data can
54 September 2007
OBD II GENERIC PID DIAGNOSIS
Here’s a basic scanner display showing OBD II generic
PIDs. Slow-changing PIDs like IAT and ECT can be fol-
lowed fairly easily in this format, but it’s difficult to spot
glitches in faster moving PIDs like Spark Advance.
This scan tool also allows the user to graph some PIDs,
while continuing to display the others in conventional
numeric format. Due to OBD II’s refresh capabilities on
some vehicles, it’s best to limit your PID choices to
those directly related to your diagnostic approach.
This photo illustrates how far PID data collection and display have come. Several
hundred thousand techs are still using the original Snap-on “brick” (on the left),
which displays a limited amount of PID data on its screen. Scrolling up or down
revealed more PIDs. The color version on the right brought graphing capability to
the brick, and extended the product’s life span by several years.
Photos:KarlSeyfert
PhotocourtesySnap-onDiagnostics

be used for comparison after
your repairs. The “before”
freeze frame shot and its PID
data establish the baseline.
As you begin your diagno-
sis, correct basic problems
first—loose belts, weak bat-
teries, corroded cables, low
coolant levels and the like.
The battery and charging sys-
tem are especially important,
due to their effect on vehicle
electronics. A good battery, a
properly functioning alterna-
tor and good connections at
power and ground circuits
are essential. You can’t as-
sume that OBD II will detect
a voltage supply problem that
can affect the entire system.
If you have an intermittent
problem that comes and
goes, or random problems
that don’t follow a logical pattern, check
the grounds for the PCM and any other
controller in the vehicle.
If the basics check out, focus your di-
agnosis on critical engine parameters
and sensors first. Write down what you
find; there’s too much information to
keep it all in your head. Add any infor-
mation collected from the vehicle own-
er regarding vehicle performance. Jot
down the battery voltage and the results
of any simple tests, such as fuel pressure
or engine vacuum. Look at the Readi-
ness Status display to see if there are
any monitors that aren’t running to
completion.
Datastream Analysis
Take your time when you begin looking
at the live OBD II datastream. If you se-
lect too many items at one time, the scan
tool update will slow. The more PIDs
you select, the slower the update rate
will be. Look carefully at the PIDs and
their values. Is there one line of data that
seems wrong? Compare data items to
one another.
Do MAP and BARO agree key on,
engine off (KOEO)? Are IAT and ECT
the same when the engine is cold
KOEO? The ECT and IAT should be
within 5°F of each other. ECT should
reach operating temperature, preferably
190°F or higher. If the ECT is too low,
the PCM may richen the fuel mixture to
compensate for a (perceived) cold-
engine condition. IAT should read ambi-
ent temperature or close to underhood
temperature, depending on the location
of the sensor.
Is the battery voltage good KOEO?
Is the charging voltage adequate when
the engine starts? Do the MAP and
BARO readings seem logical? Do the
IAC counts look too high or
too low? Compare data items
to known-good values you’d
expect to see for similar op-
erating conditions on similar
vehicles.
Check short-term fuel
trim (STFT) and long-term
fuel trim (LTFT). Fuel trim
is a key diagnostic parameter
and tells you what the com-
puter is doing to control fuel
delivery and how the adap-
tive strategy is operating.
STFT and LTFT are ex-
pressed as a percentage, with
the ideal range being within
±5%. Positive fuel trim per-
centages indicate that the
powertrain control module
(PCM) is attempting to en-
richen the fuel mixture to
compensate for a perceived
lean condition. Negative fuel trim per-
centages indicate that the PCM is at-
tempting to enlean the fuel mixture to
compensate for a perceived rich condi-
tion. STFT will normally sweep rapidly
between enrichment and enleanment,
while LTFT will remain more stable. If
either STFT or LTFT exceeds ±10%,
this should alert you to a potential
problem.
56 September 2007
The Snap-on MODIS is a combination scanner, lab/igni-
tion scope, DVOM and Troubleshooter. In scanner mode,
MODIS can graph several parameters simultaneously,
as seen in this screen capture. Remember, although
these may look like scope patterns, the reporting rate
for PID data on a scanner isn’t nearly as fast.
When scan tool screen real estate is
limited, porting the scan tool into a
laptop or desktop PC allows you to
graph more PIDs simultaneously.
The PC’s much larger memory ca-
pacity also makes it possible to col-
lect PID data in movie format for
later playback and analysis.
An on-screen description of the PID
displayed below the graphing data
may help you to understand what
you’re looking at, and avoid misunder-
standings with measurement units.
Screencapture:JorgeMenchu
PhotocourtesySPX/OTC
PhotocourtesyInjectoclean

Determine if the condition exists in
more than one operating range. Check
fuel trim at idle, at 1500 rpm and at
2500 rpm. If LTFT B1 is 20% at idle but
corrects to 5% at both 1500 and 2500
rpm, focus your diagnosis on factors that
can cause a lean condition at idle, such
as a vacuum leak. If the condition exists
in all rpm ranges, the cause is more like-
ly to be fuel-related, such as a bad fuel
pump, restricted injectors, etc.
Fuel trim can also be used to identify
which bank of cylinders is causing a
problem on bank-to-bank fuel control
engines. For example, if LTFT B1 is
Ϫ25% and LTFT B2 is 5%, the source
of the problem is associated with B1
cylinders only, and your diagnosis
should focus on factors related to B1
cylinders only.
The following parameters could af-
fect fuel trim or provide additional diag-
nostic information. Also, even if fuel
trim is not a concern, you might find an
indication of another problem when re-
viewing these parameters:
Fuel System 1 Status and Fuel Sys-
tem 2 Status should be in closed-loop
(CL). If the PCM is not able to
achieve CL, the fuel trim data may not
be accurate.
If the system includes one, the mass
airflow (MAF) sensor measures the
amount of air flowing into the engine.
The PCM uses this information to cal-
culate the amount of fuel that should be
delivered to achieve the desired air/fuel
mixture. Check the MAF sensor for ac-
curacy in various rpm ranges, including
wide-open throttle (WOT), and com-
pare it with the manufacturer’s recom-
mendations.
When checking MAF sensor read-
ings, be sure to identify the unit of mea-
surement. The scan tool may report the
information in grams per second (gm/S)
or pounds per minute (lb/min). Some
technicians replace the sensor, only to
realize later that the scan tool was not
set correctly. Some scan tools let you
change the units of measurement for
different PIDs so the scan tool matches
the specification in your reference man-
ual. Most scan tools let you switch easily
between Fahrenheit and Celsius tem-
perature scales, for example. But MAF
specs can be confusing when the scan
tool shows lb/min and we have a spec
for gm/S. Here are a few common con-
version formulas, in case your scan tool
doesn’t support all of these units of
measurement:
Degrees Fahrenheit ϪϪ 32 ϫϫ 5/9 ϭϭ Degrees Celsius
Degrees Celsius ϫϫ 9/5 + 32 ϭϭ Degrees Fahrenheit
lb/min ϫϫ 7.5 ϭϭ gm/S
gm/S ϫϫ 1.32 ϭϭ lb/min
The Manifold Absolute Pressure
(MAP) Sensor PID, if available, indi-
cates manifold pressure, which is used
by the PCM to calculate engine load.
The reading is normally displayed in
inches of mercury (in./Hg). Don’t con-
fuse the MAP sensor parameter with in-
take manifold vacuum; they’re not the
same. Use this formula: barometric
58 September 2007
Graphs aren’t the only way to display PID data. Once
transferred to the PC with its greater screen real es-
tate, PID data can be converted to formats that relate
to the data. A red thermometer scale is much easier to
follow than changing numbers on a scan tool.
PC-based scan tools excel at capturing and displaying
large amounts of PID data for later analysis. Graphing
the data, then analyzing it on-screen, may allow you to
spot inconsistencies and provides an easy method for
overlaying similar or related PID data.
Here’s a peek at some of the addition-
al PID data that’s available on late-
model vehicles. This screen capture
was taken from a CAN-enabled 2005
vehicle, and includes PIDs for EVAP
PURGE, FUEL LEVEL and WARM-UPS, as
well as familiar PIDs like BARO. This
much PID data in generic mode should
aid in diagnosis when manufacturer-
specific PID data is not available.
Screencaptures:JorgeMenchu
ScreencapturecourtesyBoschDiagnostics

pressure (BARO) Ϫ MAP ϭ intake
manifold vacuum. For example, BARO
(27.5 in./Hg) Ϫ MAP (10.5) ϭ intake
manifold vacuum (17.0 in./Hg). Some
vehicles are equipped with only a MAF
sensor, some have only a MAP sensor
and some are equipped with both.
The PIDs for Oxygen Sensor Output
Voltage B1S1, B2S1, B1S2, etc., are
used by the PCM to control fuel mix-
ture and to detect catalytic converter
degradation. The scan tool can be used
to check basic sensor operation. The
sensor must exceed .8 volt and drop be-
low .2 volt, and the transition from low
to high and high to low should be quick.
A good snap throttle test will verify the
sensor’s ability to achieve the .8 and .2
voltage limits. If this method doesn’t
work, use a bottle of propane to manu-
ally richen the fuel mixture to check the
oxygen sensor’s maximum voltage out-
put. To check the sensor’s low voltage
range, simply create a lean condition
and check the voltage.
Remember, your scan tool is not a lab
scope. You’re not measuring the sensor
in real time. The PCM receives the data
from the oxygen sensor, processes it,
then reports it to the scan tool. Also, a
fundamental OBD II generic limitation
is the speed at which that data is deliv-
ered to the scan tool. In most cases, the
fastest possible data rate is approximate-
ly 10 times a second, with only one pa-
rameter selected. If you’re requesting
and/or displaying 10 parameters, this
slows the data sample rate, and each pa-
rameter is reported to the scan tool just
once per second. You can achieve the
best results by graphing or displaying
data from each oxygen sensor separately.
If the transition seems slow, the sensor
should be tested with a lab scope to veri-
fy the diagnosis before you replace it.
The Engine Speed (RPM) and Igni-
tion Timing Advance PIDs can be used
to verify good idle control strategy.
Again, these are best checked using a
graphing scan tool. Check the RPM,
Vehicle Speed Sensor (VSS) and Throt-
tle Position Sensor (TPS) PIDs for ac-
curacy. These parameters can also be
used as reference points to duplicate
symptoms and locate problems in
recordings.
Most PID values can be verified by
a voltage, frequency, temperature,
vacuum or pressure test. Engine
coolant temperature, for example, can
be verified with a noncontact temper-
ature tester, while intake manifold vac-
uum can be verified with an accurate
vacuum gauge. Electrical values also
should be tested with a DVOM. If the
electrical value exists at the sensor but
not at the appropriate PCM terminal,
then the component might be experi-
encing a circuit fault.
Calculated Values
Calculated scan tool values can cause a
lot of confusion. The PCM may detect a
failed ECT sensor or circuit and store a
DTC. Without the ECT sensor input,
59September 2007
Circle #31

the PCM has no idea what the coolant
temperature really is, so it may “plug in”
a temperature it thinks will work to
keep the engine running long enough to
get it to a repair shop. When it does
this, your scanner will display the fail-
safe value. You might think it’s a live val-
ue from a working sensor, when it isn’t.
Also be aware that when a compo-
nent such as an oxygen sensor is discon-
nected, the PCM may substitute a de-
fault value into the datastream displayed
on the scan tool. If a PID is static and
doesn’t track with engine operating con-
ditions, it may be a default value that
merits further investigation.
Graphing Data
If you’ve ever found it difficult to com-
pare several parameters at once on a
small scan tool screen, graphing PIDs is
an appealing proposition. Graphing
multiple parameters at the same time
can help you compare data and look for
individual signals that don’t match up to
actual operating conditions.
Although scan tool graphing isn’t
equivalent in quality and accuracy to a
lab scope reading, it can provide a com-
parative analysis of the activity in the
two, three, four or six oxygen sensors
found in most OBD II systems.
Many scan tools are capable of stor-
ing a multiple-frame movie of selected
PIDs. The scan tool can be programmed
to record a movie after a specific DTC
is stored in the PCM. Alternatively, the
scan tool movie might be triggered
manually when a driveability symptom
occurs. In either case, you can observe
the data or download it and print it lat-
er. Several software programs let you
download a movie, then plot the values
in a graphical display on your computer
monitor.
Make the Most of
What You’ve Got
Take the time to learn what your scan
tool will do when connected to a spe-
cific make or model. Do your best to
gather all relevant information about
the vehicle system being tested. That
way you can get the most out of what
the scan tool and PCM have to offer.
The OBD II system won’t store a DTC
unless it sees (or thinks it sees) a prob-
lem that can result in increased emis-
sions. The only way to know what the
PCM sees (or thinks it sees) is to look
through the window provided by the
scan tool interface.
You have a DTC and its definition.
You have freeze frame data that may
help you zero in on the affected compo-
nent or subsystem. PIDs have already
provided you with additional clues
about the operation of critical sensors.
Keep your diagnosis simple as long as
you can. Now fix the car.
60 September 2007
Visit www.motor.com to download
a free copy of this article.
Circle #32
Circle #33
Circle #34
Circle #35

Photo&screencaptures:BobPattengale
52 March 2005

I
f you don’t have a good starting point, driveability diagnostics
can be a frustrating experience. One of the best places to start
is with a scan tool. The question asked by many is, “Which
scan tool should I use?” In a perfect world with unlimited re-
sources, the first choice would probably be the factory scan tool.
Unfortunately, most technicians
don’t have extra-deep pockets. That’s
why my first choice is an OBD II
generic scan tool. I’ve found that ap-
proximately 80% of the driveability
problems I diagnose can be narrowed
down or solved using nothing more
than OBD II generic parameters. And
all of that information is available on
an OBD II generic scan tool that can
be purchased for under $300.
The good news is the recent phase-in
of new parameters will make OBD II
generic data even more valuable. Fig. 1
on page 54 was taken from a 2002 Nis-
san Maxima and shows the typical para-
meters available on most OBD II-
equipped vehicles. As many as 36 para-
meters were available under the original
OBD II specification. Most vehicles
from that era will support 13 to 20 para-
meters. The California Air Resources
Board (CARB) revisions to OBD II
CAN-equipped vehicles will increase
the number of potential generic para-
meters to more than 100. Fig. 2 on page
56 shows data from a CAN-equipped
2005 Dodge Durango. As you can see,
the quality and quantity of data has in-
creased significantly. This article will
identify the parameters that provide the
greatest amount of useful information
and take a look at the new parameters
that are being phased in.
No matter what the driveability is-
sue happens to be, the first parame-
53March 2005
INTERPRETING
G E N E R I C
SCANDATABY BOB PATTENGALE
Readily available ‘generic’ scan data provides an
excellent foundation for OBD II diagnostics.
Recent enhancements have increased the value of
this information when servicing newer vehicles.

ters to check are short-term fuel trim
(STFT) and long-term fuel trim
(LTFT). Fuel trim is a key diagnostic
parameter and your window into what
the computer is doing to control fuel
delivery and how the adaptive strategy
is operating. STFT and LTFT are ex-
pressed as a percentage, with the ideal
range being within Ϯ5%. Positive fuel
trim percentages indicate that the
powertrain control module (PCM) is
attempting to enrichen the fuel mix-
ture to compensate for a perceived
lean condition. Negative fuel trim
percentages indicate that the PCM is
attempting to enlean the fuel mixture
to compensate for a perceived rich
condition. STFT will normally sweep
rapidly between enrichment and en-
leanment, while LTFT will remain
more stable. If STFT or LTFT ex-
ceeds Ϯ10%, this should alert you to
a potential problem.
The next step is to determine if the
condition exists in more than one op-
erating range. Fuel trim should be
checked at idle, at 1500 rpm and at
2500 rpm. For example, if LTFT B1 is
25% at idle but corrects to 4% at both
1500 and 2500 rpm, your diagnosis
should focus on factors that can cause
a lean condition at idle, such as a vacu-
um leak. If the condition exists in all
rpm ranges, the cause is more likely to
be fuel supply-related, such as a bad
fuel pump, restricted injectors, etc.
Fuel trim can also be used to identi-
fy which bank of cylinders is causing a
problem. This will work only on bank-
to-bank fuel control engines. For ex-
ample, if LTFT B1 is Ϫ20% and LTFT
B2 is 3%, the source of the problem is
associated with B1 cylinders only, and
your diagnosis should focus on factors
related to B1 cylinders only.
The following parameters could af-
fect fuel trim or provide additional
diagnostic information. Also, even if
fuel trim is not a concern, you might
find an indication of another problem
when reviewing these parameters:
Fuel System 1 Status and Fuel
System 2 Status should be in closed-
loop (CL). If the PCM is not able to
achieve CL, the fuel trim data may not
be accurate.
Engine Coolant Temperature
(ECT) should reach operating temper-
ature, preferably 190°F or higher. If
the ECT is too low, the PCM may
richen the fuel mixture to compensate
for a (perceived) cold engine condition.
Intake Air Temperature (IAT)
should read ambient temperature or
close to underhood temperature, de-
pending on the location of the sensor.
In the case of a cold engine check—
Key On Engine Off (KOEO)—the
ECT and IAT should be within 5°F of
each other.
The Mass Airflow (MAF) Sensor,
if the system includes one, measures
the amount of air flowing into the en-
gine. The PCM uses this information
to calculate the amount of fuel that
54 March 2005
INTERPRETING GENERIC SCAN DATA
Fig. 1

should be delivered, to achieve the
desired air/fuel mixture. The MAF
sensor should be checked for accura-
cy in various rpm ranges, including
wide-open throttle (WOT), and com-
pared with the manufacturer’s recom-
mendations. Mark Warren’s Dec.
2003 Driveability Corner column cov-
ered volumetric efficiency, which
should help you with MAF diagnos-
tics. A copy of that article is available
at www.motor.com, and an updated
volumetric efficiency chart is available
at www.pwrtraining.com.
When checking MAF sensor read-
ings, be sure to identify the unit of
measurement. The scan tool may re-
port the information in grams per sec-
ond (gm/S) or pounds per minute
(lb/min). For example, if the MAF
sensor specification is 4 to 6 gm/S and
your scan tool is reporting .6 lb/min,
change from English units to metric
units to obtain accurate readings.
Some technicians replace the sensor,
only to realize later that the scan tool
was not set correctly. The scan tool
manufacturer might display the para-
meter in both gm/S and lb/min to help
avoid this confusion.
The Manifold Absolute Pressure
(MAP) Sensor, if available, measures
manifold pressure, which is used by
the PCM to calculate engine load. The
reading in English units is normally
displayed in inches of mercury
(in./Hg). Don’t confuse the MAP sen-
sor parameter with intake manifold
vacuum; they’re not the same. A sim-
ple formula to use is: barometric pres-
sure (BARO) Ϫ MAP ϭ intake mani-
fold vacuum. For example, BARO
27.5 in./Hg Ϫ MAP 10.5 ϭ intake
manifold vacuum of 17.0 in./Hg. Some
vehicles are equipped with only a
MAF sensor, some have only a MAP
sensor and some are equipped with
both sensors.
Oxygen Sensor Output Voltage
B1S1, B2S1, B1S2, etc., are used by
the PCM to control fuel mixture. An-
other use for the oxygen sensors is to
detect catalytic converter degradation.
The scan tool can be used to check ba-
sic sensor operation. Another way to
test oxygen sensors is with a graphing
scan tool, but you can still use the data
grid if graphing is not available on
your scanner. Most scan tools on the
market now have some form of graph-
ing capability.
The process for testing the sensors
is simple: The sensor needs to exceed
.8 volt and drop below .2 volt, and the
transition from low to high and high
to low should be quick. In most cases,
a good snap throttle test will verify
the sensor’s ability to achieve the .8
and .2 voltage limits. If this method
does not work, use a bottle of
propane to manually richen the fuel
mixture to check the oxygen sensor’s
maximum output. To check the low
oxygen sensor range, simply create a
lean condition and check the voltage.
Checking oxygen sensor speed is
where a graphing scan tool helps. Fig.
3 on page 57 and Fig. 4 on page 58
show examples of oxygen sensor data
graphed, along with STFT, LTFT and
rpm, taken from two different graph-
ing scan tools.
Remember, your scan tool is not a
lab scope. You’re not measuring the
56 March 2005
Fig. 2

sensor in real time. The PCM re-
ceives the data from the oxygen sen-
sor, processes it, then reports it to the
scan tool. Also, a fundamental OBD
II generic limitation is the speed at
which that data is delivered to the
scan tool. In most cases, the fastest
possible data rate is approximately 10
times a second with only one parame-
ter selected. If you’re requesting
and/or displaying 10 parameters, this
slows the data sample rate, and each
parameter is reported to the scan tool
just once per second. You can achieve
the best results by graphing or dis-
playing data from each oxygen sensor
separately. If the transition seems
slow, the sensor should be tested with
a lab scope to verify the diagnosis be-
fore you replace it.
Engine Speed (RPM) and Igni-
tion Timing Advance can be used
to verify good idle control strategy.
Again, these are best checked using a
graphing scan tool.
The RPM, Vehicle Speed Sensor
(VSS) and Throttle Position Sensor
(TPS) should be checked for accuracy.
These parameters can also be used as
reference points to duplicate symptoms
and locate problems in recordings.
Calculated Load, MIL Status,
Fuel Pressure and Auxiliary Input
Status (PTO) should also be consid-
ered, if they are reported.
Additional OBD II
Parameters
Now, let’s take a look at the more re-
cently introduced OBD II parameters.
These parameters were added on 2004
CAN-equipped vehicles, but may also
be found on earlier models or non-
CAN-equipped vehicles. For example,
the air/fuel sensor parameters were
available on earlier Toyota OBD II ve-
hicles. Fig. 2 was taken from a 2005
Dodge Durango and shows many of
the new parameters. Parameter de-
scriptions from Fig. 2 are followed by
the general OBD II description:
FUEL STAT 1 ϭ Fuel System 1
Status: Fuel system status will display
more than just Closed Loop (CL) or
Open Loop (OL). You might find one
of the following messages: OL-Drive,
indicating an open-loop condition
during power enrichment or decelera-
tion enleanment; OL-Fault, indicating
the PCM is commanding open-loop
due to a system fault; CL-Fault, indi-
cating the PCM may be using a differ-
ent fuel control strategy due to an
oxygen sensor fault.
ENG RUN TIME ϭ Time Since En-
gine Start: This parameter may be
useful in determining when a particu-
lar problem occurs during an engine
run cycle.
DIST MIL ON ϭ Distance Traveled
While MIL Is Activated: This para-
meter can be very useful in determin-
ing how long the customer has al-
lowed a problem to exist.
COMMAND EGR ϭ EGR_PCT:
Commanded EGR is displayed as a
percentage and is normalized for all
EGR systems. EGR commanded
OFF or Closed will display 0%, and
EGR commanded to the fully open
57March 2005
Fig. 3

position will display 100%. Keep in
mind this parameter does not reflect
the quantity of EGR flow—only what
the PCM is commanding.
EGR ERROR ϭ EGR_ERR: This
parameter is displayed in percentage
and represents EGR position errors.
The EGR Error is also normalized for
all types of EGR systems. The reading
is based on a simple formula: (Actual
EGR Position Ϫ Commanded EGR) Ϭ
Commanded EGR ϭ EGR Error. For
example, if the EGR valve is command-
ed open 10% and the EGR valve moves
only 5% (5% Ϫ 10%) Ϭ 10% ϭ Ϫ50%
error. If the scan tool displays EGR Er-
ror at 99.2% and the EGR is command-
ed OFF, this indicates that the PCM is
receiving information that the EGR
valve position is greater than 0%. This
may be due to an EGR valve that is
stuck partially open or a malfunctioning
EGR position sensor.
EVAP PURGE ϭ EVAP_PCT: This
parameter is displayed as a percent-
age and is normalized for all types of
purge systems. EVAP Purge Control
commanded OFF will display 0% and
EVAP Purge Control commanded
fully open will display 100%. This is
an important parameter to check if
the vehicle is having fuel trim prob-
lems. Fuel trim readings may be ab-
normal, due to normal purge opera-
tion. To eliminate EVAP Purge as a
potential contributor to a fuel trim
problem, block the purge valve inlet
to the intake manifold, then recheck
fuel trim.
FUEL LEVEL ϭ FUEL_PCT: Fuel
level input is a very useful parameter
when you’re attempting to complete
system monitors and diagnose specif-
ic problems. For example, the misfire
monitor on a 1999 Ford F-150 re-
quires the fuel tank level to be
greater than 15%. If you’re attempt-
ing to duplicate a misfire condition by
monitoring misfire counts and the fuel
level is under 15%, the misfire moni-
tor may not run. This is also impor-
tant for the evaporative emissions
monitor, where many manufacturers
require the fuel level to be above
15% and below 85%.
WARM-UPS ϭ WARM_UPS: This
parameter will count the number of
warm-ups since the DTCs were cleared.
A warm-up is defined as the ECT rising
at least 40°F from engine starting tem-
perature, then reaching a minimum
temperature of 160°F. This parameter
will be useful in verifying warm-up cy-
cles, if you’re attempting to duplicate a
specific code that requires at least two
warm-up cycles for completion.
BARO ϭ BARO: This parameter is
useful for diagnosing issues with
MAP and MAF sensors. Check this
parameter KOEO for accuracy relat-
ed to your elevation.
CAT TMP B1S1/B2S1 ϭ
CATEMP11, 21, etc.: Catalyst tem-
perature displays the substrate temper-
ature for a specific catalyst. The tem-
perature value may be obtained directly
from a sensor or inferred using other
sensor inputs. This parameter should
have significant value when checking
catalyst operation or looking at reasons
for premature catalyst failure, say, due
to overheating.
58 March 2005
Fig. 4

CTRL MOD (V) ϭ VPWR: I was
surprised this parameter was not in-
cluded in the original OBD II specifi-
cation. Voltage supply to the PCM is
critical and is overlooked by many
technicians. The voltage displayed
should be close to the voltage present
at the battery. This parameter can be
used to look for low voltage supply is-
sues. Keep in mind there are other
voltage supplies to the PCM. The igni-
tion voltage supply is a common source
of driveability issues, but can still be
checked only with an enhanced scan
tool or by direct measurement.
ABSOLUT LOAD ϭ LOAD_ABS:
This parameter is the normalized value
of air mass per intake stroke displayed
as a percentage. Absolute load value
ranges from 0% to approximately 95%
for normally aspirated engines and 0%
to 400% for boosted engines. The infor-
mation is used to schedule spark and
EGR rates, and to determine the
pumping efficiency of the engine for di-
agnostic purposes.
OL EQ RATIO ϭ EQ_RAT: Com-
manded equivalence ratio is used to de-
termine the commanded air/fuel ratio
of the engine. For conventional oxygen
sensor vehicles, the scan tool should dis-
play 1.0 in closed-loop and the PCM-
commanded EQ ratio during open-
loop. Wide-range and linear oxygen
sensors will display the PCM-com-
manded EQ ratio in both open-loop
and closed-loop. To calculate the actual
A/F ratio being commanded, multiply
the stoichiometric A/F ratio by the EQ
ratio. For example, stoichiometric is a
14.64:1 ratio for gasoline. If the com-
manded EQ ratio is .95, the command-
ed A/F is 14.64 ϫ 0.95 ϭ 13.9 A/F.
TP-B ABS, APP-D, APP-E, COM-
MAND TAC: These parameters relate
to the throttle-by-wire system on the
2005 Dodge Durango of Fig. 2 and will
be useful for diagnosing issues with this
system. There are other throttle-by-wire
generic parameters available for differ-
ent types of systems on other vehicles.
There are other parameters of inter-
est, but they’re not displayed or avail-
able on this vehicle. Misfire data will be
available for individual cylinders, similar
to the information displayed on a GM
enhanced scan tool. Also, if available,
wide-range and linear air/fuel sensors
are reported per sensor in voltage or
milliamp (mA) measurements.
Fig. 5 above shows a screen capture
from the Vetronix MTS 3100 Mas-
tertech. The red circle highlights the
“greater than” symbol (>), indicating
that multiple ECU responses differ in
value for this parameter. The blue cir-
cle highlights the equal sign (=), indi-
cating that more than one ECU sup-
ports this parameter and similar values
have been received for this parameter.
Another possible symbol is the excla-
mation point (!), indicating that no re-
sponses have been received for this
parameter, although it should be sup-
ported. This information will be useful
in diagnosing problems with data on
the CAN bus.
As you can see, OBD II generic data
has come a long way, and the data can
be very useful in the diagnostic process.
The important thing is to take time to
check each parameter and determine
how they relate to one another.
If you haven’t already purchased an
OBD II generic scan tool, look for
one that can graph and record, if pos-
sible. The benefits will immediately
pay off. The new parameters will take
some time to sort out, but the diag-
nostic value will be significant. Keep
in mind that the OBD II generic
specification is not always followed to
the letter, so it’s important to check
the vehicle service information for
variations and specifications.
60 March 2005
Fig. 5

37August 2008
L
ast month’s installment
on datastream analysis
focused on the value of
freeze frame data, Mode
5 and Mode 6 data and
KOEO (key on, engine
off) datastream. This month’s discussion
picks up where we left off, with KOER
(key on, engine running) analysis. So go
ahead, start the engine!
I recommend that KOER data col-
lection always start in the generic, or
global OBD II interface. Why? Because
generic datastream PID values are nev-
er substitutes for actual sensor readings.
For example, you can disconnect the
MAP sensor connector on a Chrysler
product and drive it around while moni-
toring datastream in the enhanced
(manufacturer-specific) interface. (Try
this yourself; don’t just take my word for
it.) You’ll see the MAP PID change
along with the TPS sensor reading and
rpm, showing a range of values that re-
flect likely MAP readings for each con-
dition, moment by moment. These are
substituted values. If you looked at the
MAP voltage PID, however, it would
show an unchanging reference voltage.
In the enhanced interface, substitutions
can and do occur. But in the generic in-
terface, substituted values are never al-
lowed. You would see MAP shown at a
constant pressure equal to something a
DATASTREAM
IN-DEPTH
ANALYSISBY SAM BELL
We began this two-part article with a
discussion of preliminary OBD II datastream
analysis, conducted with the engine off.
We’re going deeper this time, to explain
the value of datastream information
collected with the engine running.
Photoillustration:HaroldPerry;photos:WieckMedia&JupiterImages

bit higher than BARO. The generic in-
terface allows calculated values, but
never substituted values.
So, what are we looking for, now that
we’ve finally started the engine? The
specific answer, of course, will depend
largely on the details of the customer
complaint and/or DTC(s) that are
stored. We might, for example, be focus-
ing on fuel trim numbers (and trends) if
our code suggests an underlying air/fuel
metering problem. We might be looking
most closely at engine coolant tempera-
ture, and time-until-warm measure-
ments when that seems warranted. Per-
haps our problem lies in the evap area,
or involves EGR flow. But ultimately, it
doesn’t matter what the specific issue is;
we’ll have to focus in on the systemic in-
teractions that determine the overall
characteristics of a particular data set.
Here’s a concrete example to illus-
trate what I mean. The vehicle in ques-
tion is a 1999 Chevy Venture minivan
with the 3.4L V6. There was a DTC
P0171 (Exhaust Too Lean, Bank 1) in
memory with an active MIL. The sum
of Short Term and Long Term Fuel
Trims in freeze frame was in excess of
50%. Fuel pressure and volume had
been verified as within specification.
When evaluating a fuel trim trouble
code, one of the first steps must always
be to verify that the oxygen sensor (on
which the DTC is based) is functioning
correctly. During the test drive, I ob-
served the O2 sensor switching rich, but
not as often as would be expected if the
very large fuel trim corrections shown
were actually effective. Indeed, on the
face of it, datastream seemed to confirm
the DTC. Longtime readers, however,
can probably anticipate what my next
tests were: I checked the actual lambda
value of the exhaust gases. Then I
looked for a dynamic response as I artifi-
cially enriched the system with a blast of
propane, then enleaned it by discon-
necting a major vacuum hose. (See
“What Goes In…Harnessing Lambda as
a Diagnostic Tool” in the September
2005 issue of MOTOR. Search the index
at www.motormagazine.com for all MO-
TOR magazine articles mentioned.) Hav-
38 August 2008
DATASTREAM IN-DEPTH ANALYSIS
Data collection and analysis might yield some
helpful information, if you can find the wheat
within the chaff. This is only a small portion of a
larger data set with 100 values per PID.
When evaluating a
fuel trim trouble
code, one of the
first steps must
always be to verify
that the oxygen
sensor (on which the
DTC is based) is
functioning correctly.
Chart&screencapture:SamBell

39August 2008
ing found the idle lambda at a ridicu-
lously low value of .85 (indicating a mix-
ture with 15% more fuel than needed), I
was not surprised to see that the O2 sen-
sor didn’t register a rich condition until
the engine was very nearly flooded with
propane. When I removed the purge
hose, engine rpm climbed and the en-
gine smoothed out, while lambda
marched toward the stoichiometric ideal
value of 1.00. Once the faulty O2 sensor
was replaced, all aspects of driveability
improved, and the minivan returned to
its previous fuel consumption levels.
Dynamic tests verify DTC accuracy.
In some instances, we may be able to
utilize bidirectional controls embedded
within our scan tool packages to actuate
various components. In other cases, we
may need to improvise, using signal
simulators, power probes, jumpers,
propane or just good, old-fashioned test
driving as required to initiate change
within the system we’re working on.
(I’m not saying that it will always be as
easy as it was with the Venture. You and
I know there will be problems that don’t
set DTCs, problems that do set DTCs
that have no apparent connection to the
actual root fault and, of course, prob-
lems that set appropriate codes yet are
still really hard to diagnose.)
Floodlights and Spotlights
One of the most powerful features of
most scan tools is, as nearly as I can
tell, one of the least used. This is the
so-called flight recorder, data logger or
movie mode. By whatever name it’s
known, this is an analytical tool of con-
siderable value.
Take a look at the portion of saved
scan data portrayed in the chart on page
38. As you see, any value in that infor-
mation is well hidden. This might be
termed a “floodlight” view, showing too
many values for too many parameters.
But look at the “spotlight” view above,
where I’ve selected and graphed a few
of the same PID values. This was a ve-
hicle where there was no DTC stored in
memory. By including both upstream
O2 sensors, I have provided myself a
cross-check, as there is less likelihood of
Graphical representations of scan data “movies” can speed analysis. As an added bonus,
using your scanner’s flight recorder mode allows you to concentrate on your driving. The
data set here clearly points to a lack of adequate fuel volume. This graphical representa-
tion is derived from the exact same movie capture seen in the chart on the previous page.
One of the most
powerful features
of most scan tools—
the so-called flight
recorder—seems to
be one of the least
used. But it’s an
analytical tool of
considerable value.

both being bad. Similarly, MAF and
rpm track nicely with one another, again
providing a good cross-check. The data
values at the cursor (the vertical line at
frame 2) are called out at the left side of
each PID’s plot. The upstream O2 sen-
sors are switching nicely at 2000 rpm (as
shown at frame Ϫ51), but the graphic
interface reveals an obvious problem at
higher speeds as the O2 sensors flat-line
lean. A new fuel pump restored the
missing performance.
Slow Motion and
High Speed
Moviemakers speed up or slow down
the action on the screen by shooting at
different numbers of frames per sec-
ond. When film shot at 20 frames per
second is played back at 60 frames per
second, the action seems to be occur-
ring at three times the speed. Just as a
56k dial-up modem is slower than a
DSL Internet connection, scan data
transfer rates also vary according to the
interface used. Generic communica-
tion modes often travel at a crawl, es-
40 August 2008
Most MOTOR readers have at least a passing fa-
miliarity with the concept of OBD II monitor
completion status. Even so, a brief refresher may be
in order. OBD II monitors are simply formalized sets
of self-tests all related to a particular system or
component.
CCoonnttiinnuuoouuss mmoonniittoorrss.. With a few very rare ex-
ceptions (mostly for 1998 and earlier models), the
so-called continuous monitors always show up as
“complete,” “done” or “ready.” Take this status re-
port with a grain of salt. Unplug the IAT sensor, start
the engine and check that the “Comprehensive
Component Monitor” readiness status shows com-
plete. Is the MIL on? Are there any pending codes?
How long would you have to let the vehicle idle be-
fore it will trip the MIL and show a P0113 (IAT Sen-
sor Circuit Voltage High) DTC?
As it turns out, depending on the specific make,
model and powertrain package, there are several
specific criteria that must be met before the code
will set. In one instance, the PCM must detect a VSS
signal of 35 mph or more and an ECT value of 140°F
or more, the calculated IAT must be less than Ϫ38°F
and all of these conditions must be met for at least
180 seconds of continuous duration, during which
no other engine DTCs are set—all while MAF is less
than 12 grams per second. (This particular example,
incidentally, is a two-trip code. Some other manu-
facturers may make this and other DTCs under the
component monitor’s jurisdiction into one- or two-
trip codes, sometimes with even more complicated
entry criteria.)
Continuous monitors include the comprehensive
component monitor, the fuel monitor and the mis-
fire monitor. Each monitor runs continuously when
conditions are appropriate, but not during all actu-
al driving. For example, the misfire monitor is often
suspended during 4WD operation, since feedback
through the axles over rough roads might cause
uneven disruption of the CKP signals, which could
otherwise be misidentified as misfires. Similarly, ex-
tremely low fuel tank levels may suspend both mis-
fire and fuel system monitors to avoid setting a
DTC for running out of gas.
NNoonnccoonnttiinnuuoouuss mmoonniittoorrss.. As I pointed out last
month, it’s important to note the readiness status of
the other, noncontinuous monitors as well. These are
the monitors whose status will change to “incom-
plete,” “not ready” or “not done” when the codes
are cleared. If a vehicle arrives at your shop showing
one or more incomplete monitors, it’s likely that
someone has already cleared the codes before it got
to you. (There are a few vehicles—for example, some
1996 Subarus—which may reset monitor status to in-
complete at every key-off, or other vehicles which
may have certain monitors which cannot be made to
run to completion in normal driving, such as the evap
monitor on some Toyota Paseos.) If a vehicle shows
up with incomplete monitors, however, you should
certainly document that fact on your work order and
be sure to advise the customer that there’s a very real
possibility that one or more other codes may recur af-
ter the current repair has been completed. For more
on this subject, see my article “How Not to Get MIL-
Stoned” in the April 2004 issue of MOTOR.
More importantly, for our present purposes, the
existence of incomplete monitors means that you
may not be getting the whole picture as to what
ails the vehicle you’re looking at. Keep an open
mind, remembering that there may be other, as yet
unknown issues hidden behind that incomplete
monitor, and try not to rush your diagnosis. As
mentioned in last month’s installment, there may
be some valuable data accessible via Mode 6 even
if the monitor is not complete, but there is a very
real possibility that Mode 6 data for any incom-
plete monitor may turn out to be unreliable. And,
of course, don’t overlook any pending DTCs. Re-
member, these do not illuminate the MIL, so you
must seek them out on your own.
Monitors 1.01

42 August 2008
pecially in comparison to CAN speeds.
If you’re stuck with a generic interface,
you can often accomplish more by
looking at less.
The key here is PID selection.
Choose the smallest number of PIDs
that will give you the information you
actually need. Three or four are usu-
ally sufficient. This is your version of
the filmmaker’s high-speed action
trick, as you get more updates per
unit time the fewer PIDs you select.
With several hundred possible PIDs
from which to choose, it’s just too
easy to miss an intermittent data
glitch, or to drown in a sea of too
much information (see “Live Data vs.
‘Live Data’” on page 44).
Most MOTOR readers are familiar
with the ways in which some of the ma-
jor OEMs have organized data PIDs
for display in their enhanced scan tool
interfaces. Groupings such as Misfire,
Driveability, Emissions, Accessories
and the like are good examples of the
types of data sets you may want to con-
struct while analyzing different sorts of
problems. Tracking down a nasty inter-
mittent problem? Don’t hesitate to
pare down the OEM groupings even
further to speed data updates.
Code-Setting Criteria and
Operating Conditions
If we’re trying to resolve a MIL-on
complaint, it’s critical that we first re-
view both the exact code-setting crite-
ria and the operating conditions as re-
vealed in our previously recorded
freeze frame data. We’ll need to drive
in such a way as to complete a good
“trip” so the affected monitors can
run to completion. (For a more de-
tailed discussion of OBD II trips and
monitors, see “Monitors 1.01” on
page 40.) If we fail to meet the condi-
tions under which the self-test (moni-
tor) will run, we cannot hope to make
progress. Using the previously record-
ed freeze frame parameters gives us a
good general idea of the operating
conditions required. Merely duplicat-
ing speed, load, temperature and oth-
er basic characteristics may not be
enough. This is why we need to re-
view and understand the details of the
code-setting criteria and the monitor’s
self-test strategy. For example, some
monitors cannot run until others have
already reached completion. A typical
example would be a catalytic convert-
er monitor that is suspended until the
oxygen sensor monitors have run and
passed.
Some trouble codes, or even pending
codes, suspend multiple monitors. Oth-
er vehicle faults may then go undetect-
ed until all monitors can run again. A
P0500 (VSS Malfunction) in a Corolla,
for example, will effectively suspend
even the misfire monitor.
It seems like a no-brainer: When you’re done with all
your diagnostic tests and you’ve made the necessary
repairs, you should turn off the MIL, right? That’s
what your customer probably expects, and as we all
know, meeting customer expectations is an important
part of running a successful business.
But there are often times when you should leave
the MIL on. If your area uses an OBD II “plug & play”
emissions test, the regulations usually require that no
more than one monitor can be incomplete as of the
time of testing for model year 2001 and newer vehi-
cles, with no more than two incomplete monitors for
1996 to 2000 models. In some areas, retest eligibility
requires that the converter monitor must show “com-
plete” before a retest is valid.
If an emissions test or retest is looming in your cus-
tomer’s future, you and he must work out the pros
and cons of clearing the codes and resetting the mon-
itors to “incomplete.” If you clear the codes, the mon-
itors will reset as well. This will require that someone
will have to drive a sufficient number of monitors to
completion before a retest will be valid. If local
weather conditions, for example, will prevent the
monitors from running in a timely way, your customer
might be better off if you leave the MIL on. Then your
customer would have to drive only those portions of
the drive trace needed to run the monitor under
which the current DTC set.
For example, if you’re in the frigid climes of an up-
per Midwestern winter and a customer’s vehicle failed
an emissions test because of a faulty O2 sensor heater,
you’ll both be ahead if you don’t clear the code, letting
it expire naturally as the heater monitor runs success-
fully to completion on the next two trips. This will
avoid the necessity of rerunning all the rest of the
monitors. Of course, if the vehicle failed the evap mon-
itor, you’ll be better off clearing the code, because pro-
longed subfreezing temperatures may make running
that particular monitor successfully virtually impossible
for weeks at a time.
Lights Out?

The net result is that we may have to clear the current
DTCs and extinguish the MIL before our test drive can
bear fruit. (But again, please be sure to read and record
all the freeze frame data, the status of all monitors, the list
of both current and pending DTCs and any available
Mode 6 data before clearing the MIL (see “Lights Out?”
on page 42).
We’ll need to drive long enough to let the monitors in
question reach completion. In some cases, this may require
an extended period of time. Many Ford products, for exam-
ple, normally require a minimum of a six-hour cold-soak be-
fore the evap monitor can run, although there may be ways to
force this issue in some instances. Many Chrysler oxygen sen-
sor monitors run only after engine shut-down (with key off),
so that no amount of driving will ever bring them to comple-
tion. Certain monitors, and apparently even certain scan tools,
may require a key-off sequence before the monitor status will
update from incomplete to complete. MOTOR offers an excel-
lent resource to help you understand these details—the OBD
II Drive Cycle CD Version 7.0, available from your local
MOTOR Distributor (1-800-4A-MOTOR).
In some cases, local weather conditions may make monitor
completion seem impossible until a later date, usually be-
cause of ambient temperature requirements, although some-
times as a result of road conditions. In most cases, however, it
will still be possible to complete the monitor by running the
vehicle on a lift or dynamometer. This option may occasional-
ly result in setting, say, an ABS code, but most monitors can
be run to completion swiftly and successfully on a lift. This
option may also offer a safer, faster alternative to actual driv-
ing, as trees and telephone poles are less likely to jump in
front of a vehicle on a stationary lift.
44 August 2008
DATASTREAM ANALYSIS
Circle #22
Intermittent interruptions of sensor data
can cause tricky driveability problems.
Some glitches may set a DTC while others
may not. While viewing datastream may
reveal an intermittent sensor problem, it
should not be relied upon to do so. The is-
sue, once again, is in the data rate. Even a
moderately fast interface, say the 41.6
kbps (kilobytes per second) J-1850 PWM
used on many Ford products, can easily
miss a several-millisecond dropout if it’s
not that particular PID’s turn in the data-
stream. Where symptoms or DTCs point to-
ward an intermittent sensor glitch, you’re
probably better off breaking out your
scope or graphing multimeter.
Live Data vs. ‘Live Data’

Conclusions
Proper in-depth datastream analysis
can often light the way toward correct
diagnosis of driveability concerns.
Recording all available DTCs, pending
DTCs, freeze frame data and Mode 5
and Mode 6 results before clearing any
DTCs is essential. Specific setting cri-
teria for each DTC are manufacturer-
determined, regardless of whether the
code assigned is generic or manufac-
turer-specific. Freeze frame data sets
can be used to recreate the operating
conditions under which a previous fail-
ure occurred and can help illuminate
the conditions under which certain
self-tests are conducted. Mode 5 and
Mode 6 test results can help in analyz-
ing the type and extent of certain fail-
ures. KOEO datastream analysis can
sometimes reveal sensor faults or ration-
ality concerns that might otherwise be
overlooked.
Looking at KOEO and KOER data-
stream on a regular basis makes known-
good values familiar. Once you know
the correct values, the conditions ac-
companying problems identified by
freeze frame are easier to spot. KOER
data can highlight current problems, es-
pecially when used in conjunction with
graphical scanner interfaces. Generic
data PIDs cannot include substituted
values, and so may point up faults easily
overlooked in more enhanced inter-
faces. Careful selection of custom-
grouped PIDs can provide faster scan-
ner update rates.
Pick your tools wisely. To verify hard
faults, monitor datastream as you run
actuator tests. Look for any mismatch
between the command sent to a com-
ponent and its actual response. For in-
termittent problems, record and graph
data. In tough cases, test circuits with
your scope or meter to verify actual
voltage for comparison to specs.
Used properly, these techniques
will help you arrive quickly and confi-
dently at an accurate diagnosis of the
root cause of most driveability com-
plaints.
45August 2008
Circle #23
When trying to
resolve a MIL-on
complaint, it’s
critical to first
review the exact
code-setting
criteria and the
operating
conditions as
revealed in the
freeze frame data. This article can be found online at
www.motormagazine.com.

O
nce in a while we may
encounter a total fail-
ure of a MAF sensor,
one that is, perhaps,
short circuited or inter-
nally open. Much more
common, however, are failure modes in
which the MAF sensor has become un-
reliable, underreporting or overreport-
ing the true airflow into the engine. In-
deed, as we shall see, many MAF sen-
sor failures actually result in both un-
derreporting and overreporting!
Before we get down to brass tacks, a
brief review of the basics of MAF sys-
tems is in order. Fuel control systems
for most modern gasoline engines are
centered either on MAF or MAP (man-
ifold absolute pressure). MAF systems,
which, as their name suggests, measure
the weight of incoming air and then
meter the appropriate amount of fuel to
ensure efficient combustion, are poten-
tially more precise, although MAP sys-
tems, which calculate fuel requirements
based on engine load, have historically
demonstrated greater reliability.
As you already know, combustion is
most efficient when the ratio of air to
fuel is approximately 14.7:1 by weight.
Mass and weight are essentially synony-
mous in the presence of a sufficiently
strong gravitational field such as the
Earth’s. Thus, knowing the weight of
the air entering the engine allows the
engine controller to meter the exact
amount of fuel required to achieve effi-
cient combustion. The controller com-
mands the fuel injectors to open for an
amount of time calculated to be suffi-
cient to allow the correct weight of fuel
to enter the engine, providing that the
fuel’s pressure is known. Fuel delivery is
fine-tuned by applying fuel trim correc-
tions derived from the closed-loop feed-
back of the oxygen sensor(s).
If the entire system is working as de-
signed, fuel trim corrections, expressed
as a percentage deviation from the base
fuel delivery programming, will be with-
in 10% (either positive or negative) of
the programmed quantity. In the ab-
sence of a MAF-specific diagnostic trou-
ble code (DTC), what would first lead
us to even suspect that a faulty MAF
sensor might underlie a particular drive-
ability problem?
To function correctly, all of the air
entering an engine’s combustion cham-
bers must be “seen” by the MAF sen-
sor. This means that any vacuum or air
leak downstream of the sensor will re-
sult in insufficient fuel metering, caus-
ing a lean condition in open-loop opera-
tion and higher-than-normal fuel trim
values in closed-loop. When we en-
counter a MAF sensor-equipped vehi-
cle exhibiting these symptoms, we need
to check for unmetered airflow first.
Remember, too, that unmetered airflow
may not require an external air leak. An
incorrectly applied or faulty PCV valve
can result in incorrect MAF data where
the PCV intake through the breather
hose is upstream of the MAF.
So, the first two rules of MAF sensor
diagnosis are:
1. Find and eliminate all external air
28 July 2006
MAF
DIAGNOSIS
PhotoillustrationbyHaroldPerry;photoscourtesyWellsManufacturingCorp.
SUCCESSFUL
MAFSENSOR
DIAGNOSISBY SAM BELL
A broad range of seemingly unrelated or
contradictory driveability complaints
may arise from MAF sensor
performance faults. Use this guide to
navigate out of a diagnostic thicket or,
better still, to avoid one entirely.

or vacuum leaks downstream of the
MAF sensor. When in doubt, use a
smoke machine, or lightly pressurize
the intake manifold and spray with a
soap & water solution.
2. Verify that the manufacturer-speci-
fied PCV valve is correctly installed and
functioning as designed. (This is one in-
stance where precautionary replace-
ment may be cost-justified.)
Only after these two steps have been
completed can you safely proceed with
other diagnostics. The foremost clue
that the fault lies with the MAF sensor
itself will be excessive fuel trim correc-
tions, usually negative at idle, more or
less normal in midrange operation and
positive under high load conditions (see
“How Contamination Affects Hot-Wire
& Hot-Film MAF Sensors” on page 32).
While there are several distinct MAF
sensor technologies ranging from hot-
wire or hot-film to Karman vortex and
Corialis sensors, and while MAF sensor
outputs may take the form of variable
frequency, variable current or a simple
analog voltage, the diagnostic principles
remain largely the same.
Let’s start with Ford vehicles, for a
couple of reasons. First, they are so
widespread that most of us are familiar
with them. Second, most MAF sensor-
equipped Ford products make use of a
PID (Parameter IDentification) called
BARO (barometric pressure). Up to
2001 models, this was an inferred, or
calculated, value generated by the PCM
(powertrain control module) in re-
sponse to the maximum MAF flow
rates observed on hard wide-open
throttle (WOT) acceleration. Where
this calculated BARO PID is available,
it is of great diagnostic value, since it
can confirm MAF sensor accuracy, if
only under high flow rate conditions.
To use the BARO PID, you must
first know your approximate local baro-
metric pressure. You might consult the
BARO PID on a known-good MAP
sensor-equipped vehicle. Alternatively,
your local airport can provide this data.
Do not rely on local weather stations,
however, since these usually report a
“corrected” barometric pressure. If
weather information is the only avail-
able source, a rule of thumb is to sub-
tract about 1 in. of mercury (1 in./Hg)
for every 1000 ft. of elevation above sea
level. This will yield a rough estimate of
your actual local barometric pressure.
For greater accuracy, you can purchase
a functional barometer for something
less than $40. Compare this data with
the BARO PID. A large discrepancy
here—say, more than 2 in./Hg—should
direct your suspicions toward the MAF.
Confirm your hypothesis as follows:
First, make sure you have followed the
steps outlined in the two rules above.
Next, record all freeze frame data and
all DTCs, including pending DTCs. If
the OBD monitor readiness status for
oxygen sensors shows READY, proceed
to the next step. If it doesn’t, refer to
the procedures in the following para-
graph now. Next, perform a KAM
(Keep Alive Memory) reset and drive
the vehicle. Make sure your test drive
29July 2006

includes at least three sustained WOT
accelerations. (It’s not necessary to
speed to accomplish a sustained WOT
acceleration. Rather than a WOT snap
from idle, an uphill downshift at 20 to
30 mph is usually sufficient. The WOT
prescription can be met at throttle
openings as low as 50% to 70%.) The
BARO PID should update from its de-
fault reading by the end of the third
WOT acceleration. If it’s now close to
your local barometric pressure, the
MAF sensor is not likely to be faulty. If
BARO is not close, try one of the clean-
ing techniques explained in the sidebar
“Keeping It Clean” on page 34, then
again reset KAM and take a test drive. If
the BARO is still out of range, a replace-
ment MAF sensor is in your customer’s
future. Unfortunately, in many 2002 and
later Fords, the calculated BARO PID is
supplanted by a direct BARO reading
taken from a sensor incorporated into
the ESM (EGR System Management)
valve, greatly lessening its diagnostic val-
ue for our current purposes.
If the oxygen sensor monitor status
showed INCOMPLETE above, you’ll
have to verify O2 sensor accuracy and
performance before performing the
KAM reset procedure. Use a 4- or 5-gas
analyzer to determine whether the
air/fuel ratio is correct in closed-loop
operation. The notes about lambda (␭)
below should help.
Outside of the Ford family, MAF
sensor diagnosis is more difficult. Large
fuel trim corrections—either positive or
negative—are often the only initial
pointer to MAF sensor problems.
Again, any and all air leaks downstream
of the MAF sensor must be repaired
first. Since accurate fuel trim correc-
tions depend on correct O2 sensor out-
puts, you must verify the functionality
of these sensors first. The easiest and
fastest way to do this is by checking
lambda, a type of measure of the air/fuel
ratio. (For a detailed explanation, see
my article in the September 2005 issue
of MOTOR.) If the O2 sensors are func-
tioning correctly, lambda at idle should
be very nearly equal to 1.00 in closed-
loop. You may wish to check this also at
1500 to 1800 rpm to verify adequate
mixture control off idle. Once lambda is
found to be correct, the O2 sensors are
proven good. Then any fuel trim adjust-
ments must result from unmetered or
incorrectly metered airflow or from in-
correct fuel delivery.
Distinguishing between fuel delivery
problems and MAF sensor problems
can be very frustrating. Start by verify-
ing fuel pressure and volume. (Those
who rely on pressure alone may regret
30 July 2006
SUCCESSFUL MAF SENSOR DIAGNOSIS
Fig. 1 Fig. 2
Fig. 3 Fig. 4
Screencaptures:SamBell

it.) Use your scan tool to record critical
data PIDs and graph them for analysis.
Here are a couple of examples:
In Fig. 1 on page 30, taken during a
period of closed-loop operation, short-
term fuel trims (blue and green traces)
for each bank were above 13% at 1100
rpm (red trace), yet dropped sharply
negative at 3600 rpm, proving that inade-
quate fuel delivery was not the problem.
The values indicated in the legend box-
es correspond to the readings obtained
at the indicated cursor position (vertical
black line). The vertical white line indi-
cates the trigger point for the recording.
Subsequent diagnostics focused on the
MAF sensor and the PCV system.
Take a look at the scan data graph
shown in Fig. 2. It shows a car whose
faulty fuel pump was unable to deliver
sufficient fuel under high load condi-
tions. Notice the very low O2 sensor
readings (displayed in blue) corre-
sponding to the cursor (black vertical
line just to the right of the zero time
stamp). Fuel pressure was within spec
at idle and at about 2000 rpm, but vol-
ume was very low. The sudden drop-
off in O2 activity in response to hard
acceleration is a characteristic ob-
served in many instances of MAF sen-
sor faults as well.
Ultimately, known-good snapshots,
waveforms and other data sets are in-
valuable. Take a look at the scan snap-
shot in Fig 3. Does it show good fuel
trim and appropriate MAF sensor
readings?
Since total fuel trim stays well within
the 0 ±10% range throughout the
trace, it’s a good bet that the MAF sen-
sor is working well, at least under the
sampled conditions.
How about the data set shown in
Fig. 4? In fact, the snapshot was taken
during open-loop, closed-throttle de-
celeration when fuel was not being in-
jected, so the O2 sensor PID makes
sense. It’s actually a substituted default
value inserted whenever the vehicle is
in closed-throttle decel mode. What
about the reported MAP value? A
reading of 4.00 in./Hg shows very high
engine vacuum, which jibes with the
reported TPS PID. The fuel trim data
is within the usually accepted range of
0 ±10%. Good data can come in a vari-
ety of formats.
Of course, waveform captures from
your scope are often all that are needed
to confirm a faulty MAF sensor. In our
shop, we’ve found that a snap-throttle
MAF test for Ford products should al-
ways produce a peak voltage of at least
3.8 volts DC. The snap-throttle test is
32 July 2006
Fig. 5 Fig. 6
H
ot-wire and hot-film MAF sen-
sors calculate airflow based on
monitoring the current re-
quired to maintain a constant tem-
perature in the sensing element.
When dirt accumulates, the addi-
tional surface area allows greater
heat dissipation at low airflow rates.
The dirt, however, also functions as
an insulator, with an overall net re-
sistance to heat transfer at very high
airflow rates.
At idle and under relatively low
flow/load rate conditions where the
majority of operation may take
place, the surface area effect usual-
ly predominates, causing a rich con-
dition with fuel trim corrections
usually in the range of Ϫ10% to
Ϫ5%. At sustained high flow/load
rates, the insulative effect usually
takes over, causing a lean mixture
needing fuel trim corrections as
high as +30%.
Worse still is a complex case of
“mass confusion” that may arise un-
der hard acceleration when long-
term negative fuel trim corrections,
learned in closed-loop under low-
flow-rate conditions, are applied
precisely when positive fuel trim cor-
rections would be more appropriate.
So, for example, when the system
goes to open-loop during hard accel-
eration where the MAF is already
underreporting airflow by up to
30%, the PCM may subtract an addi-
tional 10% to 15% (LTFT) from the
normal fuel delivery calculation,
leaving the system as much as 45%
leaner than desired!
In midrange operation, the two
effects (surface area and insulative
properties) may roughly cancel each
out, with fuel trims being more or
less normal. Additionally, the exact
chemistry and configuration of dirt
buildups can vary, changing the bal-
ance of power between the surface
area and insulative effects.
How Contamination Affects Hot-Wire &
Hot-Film MAF Sensors

performed the same way as
for ignition analysis. The idea
is not to race the engine, but
simply to open the throttle
abruptly to allow a momen-
tary surge of maximum air-
flow as the intake manifold
gets suddenly filled with air.
It’s critical that the throttle be
opened (and closed) as quick-
ly as possible during this test.
The waveform in Fig. 5 on
page 32 is from a known-good
MAF sensor. Note the peak
voltage of 3.8 volts. The rapid
rise and fall after the throttle
was first opened is normal
and reflects the initial gulp of
air hitting the intake manifold
walls and suddenly reaching maximum
density, greatly reducing subsequent
flow. The exact shape of the waveform
may vary from model to model, based
on intake manifold and air duct
(snorkel) design.
What’s the relationship between
MAF and engine speed? As Fig. 6
shows, rpm and airflow rate track one
another closely under the moderate ac-
celeration conditions during which this
screen capture was taken. The similarity
of the shapes of the two traces shown
here suggests, but does not prove, that
the MAF sensor is functioning well un-
der these conditions. If the airflow re-
port was consistently increased or de-
creased by the same factor, say 10% or
even 50%, the shape of its graph would
remain the same.
Consider the additional plots pre-
sented in Fig. 7 above. Does the extra
data shed any light on the MAF sensor’s
accuracy? Or is this just an example of
too much information?
Since short-term and long-term fuel
trims remain within single
digits throughout, we can be
reasonably sure that the MAF
sensor is functioning correctly.
Do we really benefit from
looking at the O2 sensor data
here? We could probably do
almost as well without it, since
we have both STFT and
LTFT, but the O2 trace (blue)
serves as an additional cross-
check on the validity of the
fuel trim calculations. More
importantly, the O2 sensor
trace proves both that an ap-
propriately rich mixture was
obtained on hard acceleration
and that applied fuel trim
corrections were effective
throughout the captured data set.
I said at the outset that hard failures
were relatively rare, but they do occur
from time to time, and I owe it to you to
discuss this type of failure as well as in-
termittent failures. Open-circuited or
short-circuited MAF sensors usually set
a trouble code, most frequently P0102
or P0103 (low input and high input, re-
spectively). P0100 is a nonspecific MAF
sensor circuit fault, while P0104 indi-
cates an intermittent circuit failure.
Checking scan data is a vital first step
toward successful diagnosis of any of
these codes. On pre-OBD II vehicles
especially, unplugging a faulty MAF
sensor will often restore a minimum de-
gree of driveability as the PCM reverts
to TPS, rpm and/or MAP as fuel deter-
minants. Certain mid-’80s GM vehicles
were notorious for intermittent MAF
sensor failures. These usually could be
easily recreated by lightly tapping with a
small screwdriver on the MAF sensor
housing at idle. A noticeable stumble
occurring with each tap clinches the
condemnation (Fig. 8, page 36).
Of course, backprobing the MAF
sensor connector for voltage drops at
both the power and ground terminals
KOER is a required step before any fi-
nal condemnation. The coincidence of
VBATT and MAF both showing 0.0
volts cannot be ignored. Neither should
the mouse nest in the MAF, nor the
gnawed wires throughout the engine
compartment.
Why is this a hard diagnosis? Conta-
34 July 2006
Fig. 7
M
ost MAF sensor failures re-
sult from contamination.
Sometimes the dirt is visible,
but more often it’s not. Technicians
have tried a variety of cleaners, with
mixed success. Many use an aerosol
brake/electrical parts cleaner, wait-
ing until the MAF sensor is cold. A
Ford trainer in my area swears by
the most popular consumer glass
cleaner. Several top technicians re-
port good results from steam clean-
ing, while others prefer a spray in-
duction cleaner.
The vast majority of technicians
warn that the MAF sensor may be
damaged by any type of cleaning
where the electrical connector is not
held upright. This is particularly true
where strong chemicals are used, as
they may pool and work their way
into the delicate electronic circuitry.
To avoid future contamination, be
wary of oiled air filters or any that
appear likely to shed lint. Poor seal-
ing of air filter housings may con-
tribute to contamination. Never
spray an ill-fitting air filter with a sili-
cone lubricant or sealer; such sprays
are likely to render the MAF sensor
inaccurate. If an engine produces ex-
cessive blowby gases, these may con-
taminate the MAF sensor, as well. Be
sure any specified filter breather ele-
ment is installed. If none is specified,
but oil accumulates in the air intake
housing, the MAF sensor or associat-
ed intake ducts, be sure to investi-
gate and remedy the cause to pre-
vent repeat failures. Be sure to check
manufacturers’ TSBs, the iATN
archives and other sources as well.
Keeping It Clean

minated MAF sensors often
overreport airflow at idle (re-
sulting in a rich condition and
negative fuel trim corrections)
while underreporting airflow
under load (resulting in a lean
condition and positive fuel
trim corrections).
This double whammy makes
diagnosis more difficult for a
number of reasons: First, many
technicians incorrectly elimi-
nate the MAF sensor as a po-
tential culprit because they ex-
pect it to show the same bias
(either over- or underreport-
ing) throughout its operating range. Sec-
ond, a lack of a direct MAF fault DTC
(such as P0100) is often mistaken to
mean that the MAF sensor must be
good. Third, the symptoms mimic
(among other possibilities) those of a ve-
hicle suffering from low fuel pump out-
put coupled with slightly leaking injec-
tors or an overly active canister purge
system. Even sluggish, contaminated or
biased oxygen sensors may cause similar
symptoms. Without appropriate testing,
it’s hard to distinguish—just by driv-
ing—among certain ignition or knock
sensor faults and MAF sensor malfunc-
tions. Additionally, since MAF sensors
are somewhat pricey, many technicians
are afraid to condemn them, fearing ei-
ther the customer’s or the boss’ wrath if
their diagnosis is not borne out. Perhaps
the biggest obstacle is lack of a
comprehensive database of
known-good waveforms, volt-
ages and scan data against
which to compare the suspect.
My own data set features
known-good scan data and
scope captures made KOEO,
at idle and on snap-throttle. In
general, these three data points
should be sufficient to identify
a faulty MAF sensor even be-
fore it sets a fuel trim code.
A bad Bosch hot-wire MAF
sensor may be the result of a
failed burn-off circuit. Don’t
simply replace the sensor; make sure
the burn-off is functional. (The purpose
of the burn-off is to clean the hot-wire
of contaminants after each trip.) Burn-
off is usually a key OFF function after
engine operation exceeding 2000 rpm.
Burn-off circuit faults may be in the
PCM or a relay. The hot-wire should
glow visibly red during burn-off.
So what can we conclude from all
this? A broad and seemingly unrelated
or even contradictory range of fuel sys-
tem-related driveability complaints may
arise from MAF sensor performance
faults. Fuel trim data showing excessive
corrections from base programming
casts strong suspicion on MAF sensor
performance issues. After recording all
DTCs and freeze frame data, many ex-
perienced techs recommend unplug-
ging a suspect MAF sensor to see if ba-
sic driveability is improved. Scope
traces at idle and on snap-throttle accel-
eration help verify MAF sensor guilt or
innocence.
As usual, a library of known-good
scan data and waveforms is invaluable.
The Min/Max voltage feature on your
DMM may not be fast enough to catch
actual peak voltage on a snap-throttle
test, but is usually sufficient for verify-
ing performance of frequency-generat-
ing (digital) MAF sensors. If your scope
is capable of pulse-width triggering, us-
ing that function will provide exact cap-
tures of digital MAF sensors in snap-
throttle testing.
36 July 2006
Fig. 8
Circle # 27

T
he Greek root gen- un-
derlies many words in
common parlance—ge-
neric is one of them.
Your medical insurance
provider and your phar-
macist both know that when it comes to
prescriptions, generic equivalents can
save us all big money, with identical re-
sults. In the last few years, generic has,
for complex reasons, become a pejora-
tive term, often used to convey the idea
of something of lesser quality than a so-
called name-brand alternative.
Yet, for diagnostic purposes, the
generic datastream sometimes offers a
better window into powertrain manage-
ment operating conditions than even
the name-brand “enhanced” or “manu-
facturer-specific” interface can. In fact,
even though I own several much more
powerful and expensive scan tools, I
routinely use the generic interface re-
siding on the cheapest of the bunch as
my go-to choice for initial code retrieval
and data analysis.
This particular machine, an aging
AutoXray EZ6000, offers no bidirec-
tional controls above code clearing, but
has the signal virtues of speed and a
very high overall connectivity rate. It al-
so quickly compiles a printable report
which includes current operating PIDs,
DTCs (including pending codes) and
freeze frame data, all of which are obvi-
ously useful. Individual monitor com-
pletion status requires a separate query,
as do both Mode $05 (oxygen sensor
test results) and Mode $06 (monitor
self-test results) data. Its nice graphing
program makes data analysis easy after
a road test, and I’m actually happy that
you cannot both read and record data
simultaneously, as the trees in my
neighborhood view that particular be-
havioral combination as an excuse to
jump out in front of you.
One of the primary benefits of the
20 July 2014
BY SAM BELL
A resourceful diagnostician knows
complicated and expensive equipment
isn’t always needed. Tools with a narrower
focus, combined with an enlightened
approach, allow him to get the job done.
BY SAM BELL
A resourceful diagnostician knows
complicated and expensive equipment
isn’t always needed. Tools with a narrower
focus, combined with an enlightened
approach, allow him to get the job done.
DOING IT ALL WITHDOING IT ALL WITHPhotoillustration:HaroldA.Perry;images:Thinkstock,DavidKimble,Sun&GeneralMotors

generic datastream stems from the re-
quirement that it not display substituted
values. While many so-called enhanced
interfaces offer access to a greater num-
ber of data PIDs, some of these may re-
flect substituted values not based on
current operating data. For example,
most Chrysler products will substitute a
reasonable guess for the actual intake
manifold vacuum value when the MAP
sensor is unplugged. If you look in the
enhanced datastream, you’ll see that
value varying quite believably as you rev
the engine or drive the vehicle. If you
look at MAP_Volts, however, you’ll see
a fixed value reflecting reference volt-
age (Vref) for the sensor circuit. But
how often do you actually look at that
PID instead of the vacuum reading?
While substituted values are prohibit-
ed in the generic datastream, calculated
values are not. Thus, for example, an
ECT PID of Ϫ40°F reflects the calcu-
lated temperature of an open ECT sen-
sor circuit. In such cases, Toyota, for ex-
ample, has for many years, then substi-
tuted a value of 176°F in its enhanced
datastream, but not in the generic data.
In our unplugged Chrysler MAP sensor
example above, using a generic inter-
face, you’d see an unmoving value of
something in the neighborhood of
255kPa or higher, corresponding to a
boost pressure of about 25 psi above at-
mospheric.
As a technical consultant to our state
EPA, several times a year I encounter
vehicles which have failed our OBD II
plug & play state emissions test for a
MIL-on condition with one or more cur-
rent DTCs that simply do not appear in
the “enhanced” interface, but which are
readily retrieved using a generic hook-
up. I’m afraid I can’t shed a lot of light
on why this would occur since, clearly, it
should not. Thus far, I have not encoun-
tered this issue in any 2008 or later vehi-
cles. There seem to be a few makes
21July 2014
GENERICDATASTREAMGENERICDATASTREAM

which are more prone to this problem,
but my data set is too sparse to be cer-
tain of any meaningful correlation. For
the moment, suffice it to say that the
state’s testing interface is also a generic
one, and, apparently, there are instances
in which a DTC may set but not be re-
trieved via even the factory scan tool. On
these occasions, only a generic interface
will work. As the saying has it, truth is
stranger than fiction.
An additional advantage of using the
generic datastream becomes apparent
when you’re working on a vehicle for
which your scan tool doesn’t provide an
enhanced interface. Don’t laugh; I’ve
had students call me up to ask what to
do because they didn’t have a scan tool
that offered, say, a Saab or Daihatsu op-
tion. A gentle reminder that they could
at least start in the generic interface
usually nets an embarrassed oops! Be-
cause the generic interface contains the
data most critical to engine operations
(see the starred items in the “Generic
PIDs” list on page 24), it’s normally suf-
ficient to rule in or rule out a particular
area of concern such as fuel delivery, for
example, early in the diagnostic process.
While you might well prefer to work
with a dealer-equivalent scan tool in al-
most all cases, in the real world you may
not be able to justify buying a tool with
limited utility vis-à-vis your regular cus-
tomer base.
Let’s take a look at what the generic
interface typically offers these days (see
the screen captures on this page). The
J1979 SAE standard specifically defines
128 generic data PIDs, but not all man-
ufacturers use or support all of them.
Some, such as Mode $01, PID$6F (cur-
rent turbocharger compressor inlet
pressure), are highly specialized and
won’t apply to most current-production
vehicles, while others, such as PID $06
(engine RPM), are pretty universal. A
typical PID list of current values (Mode
$01) or of freeze frame values (Mode
$02) would include some or all of the 74
items listed in the generic PIDs list.
Your scan tool may use slightly different
acronyms or abbreviations to identify
various data items.
Most of the PIDs in the list are prob-
ably familiar to you, but a few may have
you scratching your head. As you see,
starting with a model year 2005 phase-
in, several new parameters have been
added to the original generic data list.
These include both commanded and ac-
tual fuel-rail pressure, EGR command
and EGR error calculation, commanded
purge percentage, commanded equiva-
lence ratio and a host of others, includ-
ing many diesel-specific PIDs.
In-use counters may also indicate
how many times each of the various on-
board monitors has run to completion
since the codes were last cleared. The
list on page 24 includes most of the
generic PIDs currently in widespread
use. However, since not all manufactur-
ers support all PIDs, and since their
choices may vary by model, engine
and/or equipment, the list given here
represents only a portion of the PIDs
potentially supported. Additionally,
manufacturers are free to establish and
define supplemental modes and PIDs
which may or may not be accessible via
a generic interface. All ECUs with au-
thority or control over emissions-related
issues, however, must be accessible via
the generic interface.
From our generic PIDs list, I want to
focus on commanded equivalence ratios
first. In essence, this is the PCM’s way
of reporting how rich or lean a mixture
it’s commanding. The PID is presented
in a lambda format, with 1.0 indicating a
stoichiometric (ideal) air/fuel ratio.
Larger numbers indicate more air—a
command to run at a leaner air/fuel ra-
tio—while numbers less than 1.0 indi-
cate a correspondingly richer mixture.
If you have a gas analyzer capable of
DOING IT ALL WITH GENERIC DATASTREAM
These two screen shots show the 60 lines of generic data available from a
known-good 2014 Mazda CX-5, as captured via a Snap-on SOLUS Ultra scan tool.
22 July 2014
Screencaptures:SamBell

displaying lambda, it should coincide
extremely well with the Commanded
Equivalence PID.
As with all fuel trim-related issues, it’s
best to check this PID at idle, at about
1200 rpm and at about 2500 rpm. If
your actual tailpipe measurements don’t
coincide with the PID, be sure to check
for any exhaust leaks first. If there are
none, you’ll have to check for factors
that could account for the discrepancy,
such as fuel pressure faults, vacuum
24 July 2014
The five starred (★) critical
PIDs in the list below are
the most influential inputs. Vir-
tually all the others function
merely to fine-tune (trim) the
basic spark and fuel (base
map) commands mapped out
in response to these PIDs. The
cause of any fuel trim correc-
tions (STFT, LTFT) beyond the
range of approximately Ϯ5%
must be investigated.
Standards Compliance - such
as OBD II (Federal), OBD II
(CARB), EOBD (Europe), etc.
MIL - malfunction indicator
lamp status (off/on)
MON_STAT - monitor comple-
tion status since codes cleared
DTC_CNT - number of con-
firmed emissions-related DTCs
available for display
★RPM - revolutions per min-
ute: also, engine crankshaft (or
eccentric shaft) speed, sourced
from the CKP
★IAT - intake air temperature
★ECT - engine coolant tem-
perature
★MAP and/or ★MAF - mani-
fold absolute pressure or mass
airflow, respectively
★TPS or ★TP - throttle position
sensor, usually given as calcu-
lated percentage; see absolute
TPS below
CALC_LOAD - calculated, based
on current airflow, as percent-
age of peak airflow at sea lev-
el at current rpm, with correc-
tion for current BARO
LOOP - status: closed, closed
with fault, open due to insuffi-
cient temperature, open due
to high load or decel fuel cut,
open due to system fault
STFT_x (per bank) - short-
term fuel trim; the percent-
age of fuel added to or sub-
tracted from the base fuel
schedule (for speed, load,
temperature, etc.) in order to
achieve stoichiometry as de-
termined by the relevant
air/fuel or O2 sensor
LTFT_x (per bank) - long-term
fuel trim
VSS - vehicle speed sensor
HO2SBxSy - heated oxygen
sensor, Bank x, Sensor y, such
as B1S2 for a bank 1 down-
stream sensor
IGN_ADV - ignition timing,
measured in crankshaft de-
grees
SAS or SEC_AIR - commanded
secondary air status off/on; may
include information such as at-
mosphere, upstream or down-
stream of converter, command-
ed on for diagnostic purposes
RUN_TIME - seconds since last
engine start; some manufac-
turers stop the count at 255
seconds
DISTANCE TRAVELED WITH
MIL ON – in miles or km
FRP - fuel rail pressure relative
to intake manifold pressure
FRP_G - fuel rail pressure,
gauge reading
O2Sx_WR_lambda(x) - wide
range air/fuel sensor, bank x,
equivalence ratio (0-1.999) or
voltage (0-7.999)
EGR - commanded EGR per-
centage
EGR_ERR - deviation of sensed
or calculated position from
commanded position, percent
PURGE - commanded percent-
age
FUEL_LVL - fuel level input per-
centage; can provide especially
invaluable information in freeze
frame diagnostics of misfire
codes set under “ran-out-of-
gas” conditions; unfortunately,
not universally implemented
WARMUPS - number of warm-
ups since codes cleared; a
warm-up is an ECT increase of
at least 40°F in which the ECT
reaches at least 160°F
DIST SINCE CLR - distance
since codes cleared
EVAP_PRESS - evaporative sys-
tem pressure
BARO - absolute atmospheric
pressure (varies with altitude
and weather)
O2Sx_WR_lambda(x) - equiv-
alence ratio or current - wide
range air/fuel sensor , position x,
equivalence ratio (0-1.999) or
current(-128mAto+127.99mA)
CAT_TEMP BxSy - catalyst tem-
perature by bank and position
(may be wildly unreliable)
MON_STAT - monitor status,
current trip
CONT_MOD_V - control mod-
ule voltage; usually measured
on the B+ input for the Keep-
Alive-Memory (KAM) but may
be measured on a switched ig-
nition input line
ABS_LOAD - absolute load,
percentage, 0-25,700%
REL_TPS - relative throttle po-
sition percentage
AMB_AT or AMB_TEMP - am-
bient air temperature; where
used, usually measured in
front of the radiator, while IAT
or MAT (manifold air tempera-
ture) are usually collected in
the intake ductwork, or inside
the throttle body or intake
manifold, respectively
ABS_TPx - absolute throttle
position, percentage, sensor B
or C
APP_x - accelerator pedal posi-
tion sensors D-F
TP_CMD - commanded throt-
tle actuator percentage
MIL_TIM - time run with MIL
on, minutes
FUEL_TYP - fuel type
ETOH_PCT or ETH_PCT -
ethanol fuel %
ABS_EVAP - absolute evap
system vapor pressure, 0-
327.675kPa
EVAP_P or EVAP_PRESS - evap
system vapor pressure (gauge),
from -32,767 to +32,768Pa
STFTHO2BxS2 – short-term
secondary (postcatalyst) oxy-
gen sensor trim by bank
LTFTHO2BxS2 - long-term sec-
ondary oxygen sensor trim by
bank
HY_BATT_PCT - hybrid battery
pack remaining life, percent-
age
E_OIL_T or ENG_OIL_TEMP -
engine oil temperature
INJ_TIM - fuel injection timing,
in crankshaft degrees from
Ϫ210° BTDC to ϩ302° ATDC
FUEL_RAT - engine fuel rate in
volume per unit time—e.g.,
liters per hour, gallons per
minute, etc.
TRQ_DEM - driver’s demand
engine, percent torque
TRQ_PCT - actual engine, per-
cent torque
REF_TRQ - engine reference
torque in Nm (0 to 65,535)
TRQ_A-E - engine percent
torque data at A=idle; B, C, D,
E = defined points
AFC - commanded diesel in-
take airflow control and rela-
tive intake airflow position
EGR_TEMP - exhaust gas recir-
culation temperature
COMP_IN_PRESS - turbo-
charger compressor inlet pres-
sure
BOOST - boost pressure con-
trol
VGT – variable-geometry turbo
control
WAST_GAT - wastegate con-
trol
EXH_PRESS - exhaust pressure
TURB_RPM - turbocharger rpm
TURB_TEMP - turbocharger
temperature
CACT - charge air cooler tem-
perature
EGTx - exhaust gas tempera-
ture, by bank
DPF - diesel particulate filter
DPF_T - diesel particulate filter
temperature
NOX - NOX sensor
MAN_TEMP - manifold sur-
face temperature
NOX_RGNT - NOX reagent sys-
tem
PMS - particulate matter sensor
Generic PIDs

leaks or a biased oxygen or air/fuel sen-
sor. If you observe a close correlation
with lambda, you’ll be able to use this
PID with confidence in lieu of actual
lambda readings while conducting addi-
tional tests.
In general, you should expect this
PID to read very close to 1.00 at idle in
closed-loop operation with conventional
oxygen sensors in the upstream posi-
tions. (Wide-range air/fuel [WRAF] ra-
tio sensors may target alternate values
under various driving conditions, typi-
cally targeting a leaner mix under light-
throttle cruise, for example. Additional-
ly, vehicles using gasoline direct injec-
tion [GDI] may deviate from stoichiom-
etry even at idle or under light-throttle
cruise conditions.) Keep in mind that
the name says a lot: This PID reports
the command, not necessarily the effect
of the command.
Once in a blue moon you may find
that commanded equivalence ratio
seems to travel exactly opposite from
lambda, so that a Com_Eq_Rat of .95
corresponds to an actual lambda value of
1.05, for instance. After the one instance
in which I’ve encountered this, I eventu-
ally learned to think of the PID value as
a deviation from 1.00, then move exactly
that far in the opposite direction. (An
unfortunate computer crash led to the
loss of my notes from that vehicle, and I
can no longer remember even which
foreign nameplate make it was, much
less the year, model and engine. What I
do remember is that it sure threw me
for a loop! I also remember rechecking
this at the time with another scan tool
with the same result, so I suspect that it
was simply the result of a mistranslation
somewhere along the way, and not a tool
glitch per se.)
One more note on the commanded
equivalence ratios PID: You’ll find it in
use for diesels as well. Stoichiometric
conditions for gasoline engines result in
an air/fuel ratio of approximately 14.7:1.
The advent of oxygenated fuels has ac-
customed us to seeing lambda values
showing slightly lean, up to as high as
1.04 in some cases, with no apparent
fault. Since fuel blends vary both region-
ally and seasonally, normal values for
your area may differ. With diesels, the
ratio is closer to 14.5:1, with propane
running best at 15.7:1 and natural gas
working out to about 17.2:1. If you’re us-
ing your gas analyzer on a vehicle burn-
ing one of these fuels, you’ll have to re-
set your lambda calculations accordingly.
Most gas analyzers with a computer
plotting interface readily accommodate
multiple fuel types, usually from the
setup menu. In the case of flex-fuel
26 July 2014
Value Description
0 .........Not available
1 .........Gasoline
2 .........Methanol
3 .........Ethanol
4 .........Diesel
5...........LPG (liquid propane gas)
6...........CNG (compressed natural gas)
7 .........Propane
8 .........Electric
9..........Bifuel running gasoline
10..........Bifuel running methanol
11 ........Bifuel running ethanol
12 ........Bifuel running LPG
13 ........Bifuel running CNG
14.........Bifuel running propane
15 .........Bifuel running electricity
16.........Bifuel running electric
and combustion engine
17 ........Hybrid gasoline
18 ........Hybrid ethanol
19 ........Hybrid diesel
20 ........Hybrid electric
21.........Hybrid running electric
and combustion engine
22 ........Hybrid regenerative
23 ........Bifuel running diesel
Any other value is reserved by ISO/SAE.
There are currently no definitions for
flexible-fuel vehicles.
Fuel Type Table:
Mode $01, PID $51
OBD - on-board diagnostics.
OBD II - second-generation OBD, as
specified by SAE J1979.
EOBD - Euro-specification OBD;
slightly different from SAE-spec.
JOBD - Japanese-specification OBD;
slightly different from SAE-spec.
DTC - diagnostic trouble code; P-
codes refer to powertrain manage-
ment faults; U-codes flag communi-
cation network errors; B-codes relate
to faults in body system manage-
ment; C-codes are chassis system
based.
PDTC - Permanent DTC; one that
cannot be cleared directly via scan
tool command; such codes will self-
clear after the affected monitors
have successfully run to completion
with no further faults. PDTCs are
written into a section of nonvolatile
memory, so they persist even if the
battery is disconnected and all capac-
itors are discharged.
PID - parameter identification; a val-
ue found in current or freeze frame
data; may indicate a sensor reading,
calculated value or command status.
In a nongeneric (enhanced) interface,
may indicate a substituted value.
$- or -$ - prefix or suffix indicating
that an alphanumeric string is hexa-
decimal (presented in base 16.) The
J1979 specifications which establish
the OBD II protocol are written using
hexadecimal notation throughout.
Datastream - a set of PID values, DTCs,
test results and/or PDTCs; the display
of such data on or via a scan tool.
Freeze frame - a set of PID values in-
dicating then-current data written
into the PCM’s memory when a DTC
sets, similar to an aircraft flight
recorder. Note: Freeze frame data is
erased when codes are cleared; be
sure to read and record before clear-
ing DTCs.
CAN - controller area network; also,
communication via the same.
Monitor - one or more self-tests exe-
cuted by the OBD system to deter-
mine whether a specific subsystem is
functioning within normal limits.
Monitor status changes to incom-
plete or “not done” when DTCs are
cleared, and returns to complete or
“done” once all relevant self-tests
have been run. A monitor status
showing completion is not a guaran-
tee of a successful repair unless there
are no codes and no pending codes,
and unless the vehicle has been oper-
ated under conditions similar to
those under which a previous fault
had occurred (see freeze frame).
Glossary

cars, check the ETOH_PCT PID to
help your analyzer figure out the cor-
rect stoichiometric ratio. Once you’ve
made the proper selection, you can
work from lambda without bothering to
know or remember the exact stoichio-
metric ratio involved.
I was certainly glad to see the appear-
ance of purge data in the generic list, as
knowing the commanded purge status
can assist in diagnosing several types of
driveability faults above and beyond
evap leaks and malfunctions. Remem-
ber, however, that this PID reflects only
the current commanded state, not nec-
essarily what’s actually happening.
The PIDs for EGR Command and
EGR Error are likewise helpful. De-
pending on the interface you use, how-
ever, EGR_Error may be reported
“backwards,” with 100% indicating that
command and position are in complete
agreement and 0% indicating that one
shows wide-open while the other
shows shut. (I’ve seen this on numer-
ous Hondas, where a 99.5% “error” ac-
tually meant that the valve was closed
as commanded.) As usual, a few min-
utes checking known-good vehicles
can help avoid many wasted hours
hunting problems that aren’t really
there.
Other new PIDs inform us of the
mileage since the last time the codes
were cleared as well as the distance
driven since the MIL first illuminated
for any current codes. Both of these
pieces of information can be useful, es-
pecially if yours is not the first shop to
look at a particular problem. In the case
of intermittent faults, they can also help
give you a better idea of just how fre-
quently the issue does arise.
Beyond PIDS
Potentially both more helpful and more
problematic are the new Permanent
DTCs found in mode $0A. These can-
not be cleared directly via a scan tool,
but will be self-erased once the corre-
sponding monitors have successfully
run to completion. Attempts to circum-
vent plug & play emissions tests by sim-
ply clearing codes without fixing the
underlying causes led to the develop-
ment of these Permanent DTCs. While
there are times when I would rather
just “kill the MIL,” the PDTCs make
me take the extra time to more fully ed-
ucate my customers and to verify the
efficacy of my repairs, often by resort-
ing to Mode $06 data analysis.
The key thing to remember when
working with Mode $06 data is that it’s
entirely up to the OEM to define all
TID$, CID$, MID$, etc. These defini-
tions can vary by year, engine, model
and/or equipment even within the
same OEM division, so be sure to veri-
fy the accuracy of any information
you’re using to interpret this data be-
fore you get yourself in trouble. Also
remember that many manufacturers
28 July 2014
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