P0420
P0420 Catalyst Efficiency Bank 1
P0420 means the downstream oxygen sensor pattern suggests the bank 1 catalytic converter is not storing oxygen properly.
How urgent is P0420?
The car may drive normally, but ignoring misfires or fuel-control issues can ruin a new converter too.
Common symptoms
- Check engine light
- Usually no drivability issue
- Failed emissions test
- Sulfur smell in some cases
- Reduced power if converter is clogged
Likely causes
- Worn catalytic converter
- Upstream exhaust leak
- Aging oxygen sensor
- Recent misfire damage
- Rich or lean running condition
What to check before replacing parts
1
Check for misfire and fuel trim codes first
2
Inspect for exhaust leaks
3
Compare upstream and downstream O2 sensor activity
4
Check converter temperature/performance
5
Do not replace the converter until root causes are ruled out
Use the code with symptoms
A code points at the system, not always the failed part. Add your vehicle, mileage, symptoms, and whether the check engine light is steady or flashing so the AI diagnostic can narrow the next step.