Oxygen (O2) sensor
A sensor in the exhaust that tells the engine computer whether it's burning too much or too little fuel.
In plain English
An oxygen sensor screws into the exhaust pipe and measures how much oxygen is left in the exhaust. The engine computer uses that reading to fine-tune how much fuel it sprays. Most cars have at least two: one before the catalytic converter and one after it.
Why it's on your estimate
Sensors slow down and fail with age. A bad one usually turns on the check engine light and can make the car use more fuel or run roughly.
How urgent is it?
Can usually wait
The car is normally safe to drive for a short while, but running with a bad sensor for a long time wastes fuel and can eventually harm the catalytic converter.
Questions to ask the shop
- Which sensor is it: before or after the converter, and which side of the engine?
- Did you test the sensor, or is that based only on the code?
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