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What is an 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.