OBD-II · Engine and powertrain (P)
P0420
Catalyst efficiency below threshold bank 1
What it means
The ECU's catalyst monitor reports conversion efficiency below the required threshold on bank 1. The test method depends on the engine and aftertreatment system.
Symptoms
- engine warning light
- usually no change in how the car drives
- fails the emissions part of the roadworthiness test
- sometimes a rotten-egg smell or a rattle from a broken catalyst core
Likely causes, most likely first
| No. | Cause |
|---|---|
| 01 | aged catalytic converter |
| 02 | exhaust leak |
| 03 | lazy downstream oxygen sensor |
| 04 | misfire damage |
| 05 | oil or coolant contamination |
| 06 | non-type-approved replacement catalyst |
What to check, step by step
- 01Fix any misfire, fuel-trim or lambda sensor codes first: they can damage the catalyst or fake this code
- 02Check for exhaust leaks between the upstream and downstream lambda sensors
- 03Run the OEM catalyst monitor/test under its specified conditions; account for the fitted lambda sensor types rather than treating every upstream sensor as a switching narrowband sensor.
- 04For diesel applications, follow the DOC-specific diagnostic procedure, including exhaust-temperature sensing and dosing checks where specified; petrol lambda-switching tests do not apply.
- 05Check for oil burning or coolant loss that poisons the catalyst
- 06Replace with a type-approved catalyst for that car (ECE R103 marking in the EU), not a cheap universal unit