Oxygen Sensor

          Oxygen Sensor

           An oxygen sensor (O2 sensor) measures the amount of oxygen in the exhaust gases, sending the signal to the engine computer. A front oxygen sensor is installed in the exhaust manifold or in the front exhaust pipe before the catalytic converter. As you know, a catalytic converter is a major part of the vehicle's emission control system. 

A rear oxygen sensor is mounted in the exhaust pipe after the catalytic converter. See the photo showing how the rear oxygen sensor looks inside the exhaust.
Cars with a 4-cylinder engine have at least two oxygen sensors; V6 and V8 vehicles have at least four O2 sensors.
The engine computer (Powertrain Control Module, or PCM) uses the signal from the front oxygen sensor to adjust the air/fuel ratio by adding or subtracting fuel. The rear oxygen sensor signal is used to monitor the performance of the catalytic converter