OBD codes / P0420
P0420
ModerateCatalyst System Efficiency Below Threshold (Bank 1)
Get it looked at soon to avoid further wear.
Likely causes
- aging catalytic converter
- faulty O2 sensor
- exhaust leak
- upstream misfire
Common fixes
- verify O2 sensors
- fix misfires first
- inspect exhaust
Seeing P0420 on your car? Get a diagnosis specific to your exact year, make and model.
Diagnose P0420 for my vehicleGeneric OBD-II reference. Manufacturer-specific behavior varies — confirm with a scan tool and, for safety-related codes, a professional inspection.