Typical Oxygen Sensor Locations
See Location Picture below.
An oxygen sensor, also known as an O2 sensor is a common part to fail and cause a check engine light on your car or truck. This sensor measures the oxygen levels in the exhaust stream to determine if the engine is running rich or lean. The computer then uses this information to recalculate the fuel delivery depending on what is needed.
If the sensor sees that the engine is running rich (too much as) then it will adjust the fuel injector on and off time to correct the condition. The same is true for a lean condition (not enough gas). The computer is constantly trying to regulate fuel flow to maintain a fuel to air ration of 14.7 to 1. This has been determined years ago as the perfect ratio for proper engine idle, acceleration and emissions.
As the years have gone by and the demand for better fuel economy has risen, along with very strict federal emissions standards, car manufacturers have added more and more oxygen sensors. We now have up to 4 sensor on a given vehicle. There is one on each exhaust manifold, one after those two pipes converge, and one after the catalytic converter. This is called the post converter sensor. This is used to measure the efficiency of the converter. If the last)2 sensor sees that the converter is not doing it's job well enough, it will turn on the check engine light and give a code of P0420.