What P0420 means
The downstream oxygen sensor is mirroring the upstream one, meaning the catalytic converter is no longer storing oxygen as it should.
Common causes
- Aged or damaged catalytic converter
- Exhaust leak before or between the sensors
- Failing downstream oxygen sensor
- An untreated misfire or rich condition that poisoned the catalyst
How it is usually fixed
- 1Fix any misfire or fuel-trim fault first — replacing a catalyst without fixing the cause will destroy the new one
- 2Check for exhaust leaks
- 3Compare upstream and downstream sensor waveforms before condemning the catalyst
Other common codes
- P0011 — Camshaft Position A – Timing Over-Advanced (Bank 1)
- P0030 — HO2S Heater Control Circuit (Bank 1, Sensor 1)
- P0101 — Mass Air Flow Circuit Range/Performance
- P0113 — Intake Air Temperature Sensor Circuit High
- P0118 — Engine Coolant Temperature Sensor Circuit High
- P0131 — O2 Sensor Circuit Low Voltage (Bank 1, Sensor 1)
Code information is general guidance, not a diagnosis for a specific vehicle. Confirm with live data and manufacturer test procedures before replacing parts.