A five gas analyzer is a device that detects and analyzes five separate components of an automotive exhaust: carbon monoxide (CO), carbon dioxide (CO2), oxygen (O2), hydrocarbons (CH compounds), and oxides of nitrogen (including NO and NO2). Three of these - carbon monoxide, hydrocarbons, and nitrogen oxides are pollutants. Although the other two gases - carbon dioxide and oxygen - are non-regulated, their measurement can provide a mechanic with additional information for diagnosis of engine problems.

5 gas analyzers are valuable tools following engine repairs and tune-ups. They are also useful in detecting and tracking down fuel, exhaust, emission control, and engine service problems.

High or low readings of any of the five gas components can point to a variety faults in the vehicle. For example, if the analyzer detects higher than normal hydrocarbon concentrations in the exhaust mix points to an excessive amount of unburned fuel due to incomplete combustion or a lack of ignition. The root causes of the problem could include dirty spark plugs, damaged ignition wires, and a vacuum leak, among others. Elevated nitrogen oxide levels might point, for example, to a faulty catalytic converter or a stuck thermostatic air heater. By looking at the clues offered by measurements on the different gas components, it is usually possible to isolate the problem to a small number of possibilities, which can then be individually checked.