OBD-II code guide
P0420 Catalyst System Efficiency Below Threshold
P0420 means the catalytic converter is not storing oxygen as expected, or the sensor data makes it appear that way.
What it means
The downstream oxygen sensor pattern looks too similar to the upstream sensor pattern.
Common symptoms
- Steady check engine light
- Failed emissions test
- Rattling exhaust in some cases
- Reduced power if converter is restricted
Likely causes
- Aging catalytic converter
- Exhaust leak
- Misfire damage
- Faulty oxygen sensor data
- Rich or lean running condition
First checks
- Fix misfire or fuel trim codes first.
- Inspect for exhaust leaks.
- Check whether the converter rattles.
- Review upstream and downstream oxygen sensor behavior.