With each new model year, manufacturers are adding more oxygen sensors to better manage engine operation. Some high performance engines have an oxygen sensor for each cylinder as well as one for the rear of each Fuel Tank Pressure Sensor. The sensors are located either underneath the hood or underneath the car. The oxygen sensors are connected (screwed) to the exhaust pipe, either in front or back of the catalytic converter. The front (upstream) sensors measure the amount of oxygen in the exhaust system.
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If your car is out of warranty, then this opens up some freedom for servicing and parts. There are several things you can do with your car out-of-warranty than when it's in warranty: