Emissions
Oxygen Sensor (O2 / Lambda)
The oxygen sensor measures exhaust oxygen so the ECU can fine-tune the air-fuel mixture.
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Emissions flow
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What it does
- Reads oxygen in the exhaust stream
- Upstream sensors control mixture
- Downstream sensors monitor the catalyst
- Heated sensors warm up fast
When it fails
- Check engine light
- Worse fuel economy
- Failed emissions test
- Rich or lean codes (P0130–P0160)
How to diagnose
- Watch O2 voltage switching (0.1–0.9 V up to 10×/sec)
- Check the heater circuit resistance
- Inspect for exhaust leaks before the sensor
Maintenance
- Replace on code, not on schedule
- Use quality sensors with correct connectors
- Fix exhaust leaks to protect sensor readings
Use the diagnosis order. When a part fails, confirm each hypothesis with measurements before replacing anything — codes and symptoms overlap.
Related part
Catalytic Converter
The catalytic converter turns harmful exhaust gases into less harmful ones.