Severity: Moderate
Drivable, but it can cause more damage or poor economy. Get it checked within days.
What P0420 means
The downstream oxygen sensor shows the catalytic converter on bank 1 is not storing oxygen the way a healthy converter does. The converter may be worn out, but the code is also set by exhaust leaks and sensor problems.
Symptoms
- Usually none
- Failed emissions test
- Rotten-egg smell in some cases
- Rattle from under the vehicle if the substrate broke
Common causes
Most likely first
- 1Aged or damaged catalytic converter
- 2Exhaust leak ahead of or near the downstream O2 sensor
- 3Lazy upstream or downstream O2 sensor
- 4Engine misfire or oil burning that damaged the converter
- 5Converter overheated by a rich condition
How we diagnose it
- Fix any misfire or fuel trim codes first
- Look for exhaust leaks with the engine cold
- Compare upstream and downstream O2 activity in live data
- Check for a factory bulletin: some vehicles have a software update for this code
Every vehicle sets codes under its own conditions. We check the factory diagnostic steps and service bulletins for your VIN, so the repair targets the actual cause.