P0420 Catalyst System Efficiency Below Threshold (Bank 1)
The catalytic converter on bank 1 is no longer storing and converting oxygen as well as it should, based on the downstream oxygen sensor.
Common causes
- Worn catalytic converter
- Exhaust leak near the sensors
- Faulty downstream O2 sensor
- Engine running rich or misfiring for a long time
Symptoms
- Check engine light
- Usually no driveability change
- Possible failed emissions test
What to check
- Fix any misfire or fuel trim problem first
- Check for exhaust leaks
- Compare upstream and downstream O2 signals
- Replace the converter only after other causes are ruled out
Manufacturers may add their own detail to generic codes. If your vehicle shows a different description for P0420, follow the service information for that make and model.