What is a Rear Oxygen Sensor?
This might blow your mind but…oxygen sensors (O2s) are sensors that measure oxygen. Yep, the sensors calculate the amount of oxygen in the exhaust stream, then share that information with your car’s main computer.
Most modern cars have at least two oxygen sensors: one before the catalytic converter and one after the catalytic converter. In case you were wondering, the catalytic converter, which is located in the exhaust system, is an emissions control device that cleans up the fumes leaving your engine.
Anyhow, the front oxygen sensor (also known as an upstream oxygen sensor) is a key input to your car’s computer for fuel control. The computer uses information from the sensor to estimate the engine’s air-fuel mixture. Then, it adjusts fuel delivery accordingly.
On the other hand, the rear oxygen sensor (also known as a downstream oxygen sensor) primarily monitors the performance of the catalytic converter. The rear sensor has very little, if any, input regarding fuel control.
Oh, and one more thing – most modern oxygen sensors have built-in heaters. When you first start your engine when it’s cold, the heater element gets the sensor nice and toasty. That allows the sensor to get up-to-speed and begin operating more quickly.