Exhaust gas recirculation (EGR) is a system used in many automobiles to reduce polluting emissions. When exhaust gases are recirculated back into the engine, they help to cool the combustion chamber and prevent knocking. EGR also reduces the formation of NOx pollutants.
To make sure the EGR system is working properly, it needs to be monitored. One way this is done is by using an oxygen sensor in the exhaust stream. The oxygen sensor measures the amount of oxygen in the exhaust gases. If there is less oxygen than there should be, it means that the EGR system is not functioning correctly and needs to be checked.