A four gas analyzer is an exhaust gas analyzer that measures the levels of four types of gas, CO (carbon monoxide), HCs (hydrocarbons), O2 (oxygen), and CO2 (carbon dioxide) in the tail-pipe emission of a vehicle.

Exhaust gas analyzers work by testing a tiny part of the exhaust gases using a sampling probe. Most portable gas analyzers used an infrared technique that isolates the various emissions by temperature. A four-gas analyzer, unlike a five-gas analyzer, does not detect nitrogen oxides.