This photo shows one of the oxygen sensors (arrowed) used in a Holden VT Commodore. The VT's V6 engine has two such sensors - one for each cyclinder bank.

Your car engine’s air/fuel ratio not only has a considerable bearing on its performance but also on fuel consumption and air pollution. If the mixture is too rich (ie, too much fuel), then fuel economy will suffer and the unburnt hydrocarbons will cause air pollution. Conversely, a lean mixture (ie, too much air) will give poor engine performance and produce more nitrous-oxide pollutants.

A lean mixture can also cause serious engine damage under certain circumstances, particularly at high RPM or under heavy loads.

To combat this, all modern cars use at least one exhaust gas oxygen (EGO) sensor which is mounted on the exhaust manifold. This monitors the resultant oxygen content in the exhaust and provides a voltage output which indicates whether the mixture is rich or lean or at the "stoichiometric" point (ie, when there is just sufficient oxygen in the air-fuel mixture to give complete combustion).

The 1997 Suzuki Vitara uses a 4-wire oxygen sensor - two for the heater, one for the signal and the other for ground. It's mounted on the exhaust manifold.

This information is fed to the engine management computer (ECU) which in turn controls the fuel injectors. It enables the ECU to continuously adjust the mixture to provide optimum power and economy, consistent with low exhaust emissions.

In addition, your car’s catalytic converter has an important role to play in reducing emissions. This is also mounted in the exhaust system and converts combustion byproducts such as carbon monoxide (CO) to carbon dioxide (CO₂), unburnt hydrocarbons to CO₂ and H₂O (water) and nitrous oxide (NO) to nitrogen (N₂). Some cars include another EGO sensor after the catalytic converter, to monitor its performance.

In practice, a catalytic converter works best when the air/fuel mixture is kept within a narrow range close to the stoichiometric ratio. This ratio varies according to the fuel used but is generally 14.7:1 for unleaded petrol; ie, the air mass must be 14.7 times the fuel mass.