P0420: Catalyst System Efficiency Below Threshold (Bank 1)
Severity: Moderate
What it means
The catalytic converter isn't cleaning exhaust as well as it should, based on the oxygen sensor readings.
Common symptoms
- Check engine light, often with no driving change
- Failed emissions test
- Sometimes a rotten-egg smell
Common causes
- Worn catalytic converter
- Faulty downstream oxygen sensor
- Exhaust leak before the sensor
- Engine misfires damaging the converter
What to check first
- Fix any misfire codes first
- Listen for exhaust leaks
- Don't replace the converter before ruling out the sensor
Causes vary by vehicle. Confirm with testing before replacing parts.
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