Can I keep driving?
Generally safe to drive short-term, but the car will fail New Jersey inspection with the light on. Fix the root cause first or a new converter can fail again.
What it means
The downstream oxygen sensor shows the catalytic converter is no longer cleaning the exhaust well enough. P0420 is bank 1, P0430 is bank 2.
Common symptoms
- Check engine light
- Failed emissions inspection
- Sometimes a rotten-egg smell or rattle from under the car
Common causes
- Worn catalytic converter
- Faulty downstream oxygen sensor
- Exhaust leak near the sensors
- Earlier misfires or oil burning that damaged the catalyst
How we diagnose it
- Compare upstream and downstream O₂ sensor signals
- Check for exhaust leaks
- Look for misfire history and oil consumption that would kill a new converter
- Confirm with a temperature test across the converter
Where we see it most: All makes; common on higher-mileage BMW, Mercedes-Benz and VW
A code points to the system that reported a problem, not the exact failed part. Replacing parts based on the code alone often wastes money — testing finds the real cause.