P0420
GENERIC DTC · Emissions
Catalyst system efficiency below threshold — bank 1
What the code means
The controller sees downstream oxygen-sensor behavior inconsistent with expected catalyst oxygen storage.
Severity and driving
Service soon
Plausible causes
- Aged catalyst
- Exhaust leak
- Persistent misfire or mixture problem
- Oxygen-sensor signal fault
Next checks
- Resolve active misfire or fuel-trim codes first
- Check for exhaust leakage professionally
- Compare upstream/downstream sensor data
Source and boundary
Code structure follows SAE J2012. Manufacturer-specific definitions, pinpoint tests, wiring, torque, fluids, and repair procedures require authoritative information for the exact vehicle.
SAE J2012 Diagnostic Trouble Code DefinitionsReviewed 2026-08-13