Symptoms of P0420
- Check-engine light, usually with no change in how the car drives.
- A failed emissions test.
- Sometimes a faint sulphur / rotten-egg smell.
- Rarely, a rattle from inside the converter.
Common causes of P0420
- A worn or failed catalytic converter on bank 1.
- A lazy or failed downstream oxygen sensor.
- An exhaust leak upstream of the converter.
- An underlying misfire or fuel-trim fault that damaged the cat.
Useful checks before replacing parts
- Inspect for exhaust leaks, sensor wiring faults, and incorrect oxygen or air-fuel sensor readings before condemning the catalyst.
- Review fuel trims, misfire history, oil consumption, and coolant contamination because upstream problems can damage or mislead catalyst monitoring.
- Confirm that the catalyst monitor has run under valid conditions after any repair.
Related OBD-II codes
These nearby emissions entries can help compare wording and pin down the fault area.