The document discusses using 5 gas analysis to diagnose engine issues by looking at the relationships between oxygen (O2), carbon monoxide (CO), carbon dioxide (CO2), hydrocarbons (HC), and nitrogen oxides (NOx). Elevated levels of certain gases can indicate problems like a rich or lean mixture, misfiring, or an overloaded catalytic converter. Analyzing the 5 gas content allows technicians to quickly isolate problems, repair or replace faulty components, and get customers back on the road.