P0420 · Exhaust emission controls

Catalyst System Efficiency Below Threshold (Bank 1)

Moderate: fix within weeks

Drivable; the car will fail an OBD emissions inspection, and the cause behind a worn converter should be found first.

Likely causes, most frequent first

1. Catalytic converter worn out by age or by an engine fault (most frequent)
2. Downstream oxygen sensor reading wrong (frequent)
3. Exhaust leak near the sensors (frequent)
4. Misfire, oil burning or coolant loss that damaged the converter (frequent)