P0420Catalyst System Efficiency Below Threshold Bank 1
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- The engine module checks whether the catalyst of bank 1 still converts enough. On petrol engines it estimates the oxygen storage capacity by comparing the sensors before and after the catalyst – a good catalyst smooths the rear signal. GM uses a test on overrun (ratio below 0.35 sets the code on the first trip), Ford an index ratio above 0.75 or fuel integration after fuel cut-off. On diesels P0420 concerns the oxidation catalyst, checked by its heat release during filter regeneration (GM Duramax LML, Ford below 45 % of the expected heat). Ford names oil-borne poisoning as the most common cause.
- What you may notice
- Check engine light on, otherwise often no change in driving
- Emission test failed (higher HC, CO, NOx)
- Rattling or loss of power if the catalyst has broken up
- Likely causes
- LikelyCatalyst aged or poisoned (oil consumption, silicone, coolant)
- PossibleCatalyst overheated by misfire or rich running
- PossibleRear oxygen sensor sluggish, dirty or unsuitable
- PossibleExhaust leak ahead of or at the rear sensor
- PossibleDiesel: oxidation catalyst sooted or poisoned, post-injection faulty
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Often possible to do yourself.Typical cost: €80 – €1,800
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.