1- Sensor #1 (pre-cat.)
2- Sensor #2 (post cat.)
1- Bank 1 sensor 1
2- Bank 2 sensor 1
3- Bank 1 sensor 2
Typical v-6 engine
Typical 2 into 1 truck v-6 or v-8
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 P)420.
Se the picture below for a general view of sensor locations.
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Engine Operation
2.2L  3100 / 3400 
3.5L  3800 
4200  4.3L Vortec

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Typical Oxygen Sensor Locations