What Are the Symptoms of a Failing Oxygen Sensor?
The oxygen sensor measures the oxygen in the exhaust so the computer can tune the fuel mixture. When it weakens, the mixture is thrown off, fuel use rises and response drops.
Symptoms
A noticeable rise in fuel use, rough idle, weak acceleration, sometimes a strong exhaust smell, and a check engine light with codes like P0131, P0135, or P0420.
Why it fails
Age (it weakens gradually after ~150,000 km), poor fuel or an oil or coolant leak into the exhaust, or contamination. The upstream sensor (before the converter) is the one that tunes the mixture.
How to confirm
Read the live sensor value via OBD2: a healthy upstream sensor switches quickly between rich and lean. A slow or flat sensor is weak. Mowtar AI shows this reading and links it to the codes.
FAQ
How many oxygen sensors does a car have?
Usually two per cylinder bank (upstream and downstream); V6 and V8 engines may have four.
Can I drive with a bad sensor?
Yes but with higher fuel use, more emissions, and a risk to the converter — better to replace it soon.