Oxygen sensor
Reads the exhaust so the computer can mix fuel correctly.
Signs it's worn: Check-engine light, bad mileage, failed smog check.
What's wrong with my car? · Engine
Check-engine light on and worse gas mileage?
Usually: Oxygen sensor.
A steady check-engine light with worse gas mileage is very often an O2 sensor. Get the code read for free at most parts stores to confirm.
The parts that fix it
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This is a first read from the symptom, not a diagnosis of your car. If you're not sure, have a mechanic confirm before you buy, and bring the prices you found here with you.