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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.
Symptoms of a failing Rear Oxygen Sensor
Check engine light
If your car’s rear oxygen sensor isn’t doing its job, chances are, the computer will notice and turn on the check engine light. The device will also store corresponding diagnostic trouble codes (DTCs) in its brain. Your mechanic can retrieve said codes using an electronic device, called a scan tool.
Failed emissions test
Is it time for your routine emissions test? Well, guess what – you’re not going to pass if the check engine light is on from a bad rear oxygen sensor. You also won’t pass if the rear oxygen sensor is out to lunch and it’s preventing your car’s computer from running its system self-tests.
How urgent is a Rear Oxygen Sensor Replacement?
The rear oxygen sensor is less of a troublemaker than the front oxygen sensor. For the most part, the rear oxygen sensor just polices the catalytic converter. So, it’s unlikely to leave you stranded and it shouldn’t cause any additional harm to your vehicle.
But it will keep that annoying check engine light illuminated and, really, do you want to look at that thing every day? Plus, a bad rear oxygen sensor can cause you to fail a state emissions test – and nobody wants that.