OBD CodesWhat it means
The catalytic converter cleans the exhaust. The computer compares the oxygen sensor before it with the one after it. If the second sensor starts to copy the first, the converter is no longer storing oxygen and cleaning as it should. P0420 is one of the most common codes of all.
Codes in this group
| Code | Meaning |
|---|---|
| P0420 | Catalytic converter efficiency below limit, bank 1 |
| P0421 | Warm-up catalytic converter efficiency below limit, bank 1 |
| P0422 | Main catalytic converter efficiency below limit, bank 1 |
| P0423 | Heated catalytic converter efficiency below limit, bank 1 |
| P0424 | Heated catalytic converter temperature below limit, bank 1 |
| P0430 | Catalytic converter efficiency below limit, bank 2 |
| P0431 | Warm-up catalytic converter efficiency below limit, bank 2 |
| P0432 | Main catalytic converter efficiency below limit, bank 2 |
| P0433 | Heated catalytic converter efficiency below limit, bank 2 |
| P0434 | Heated catalytic converter temperature below limit, bank 2 |
Bank 1 is the side of the engine that has cylinder 1. Sensor 1 is before the catalytic converter and sensor 2 is after it. More terms explained.
What you may notice
- Often none besides the warning light
- Rotten-egg smell
- Loss of power if the converter is blocked
- Rattle from under the car if its core has broken up
Common causes
Most likely first:
- A worn-out catalytic converter, from age or distance
- Damage from earlier misfires or a rich mixture
- Oil or coolant being burnt and coating the converter
- An exhaust leak near the sensors
- A faulty rear oxygen sensor giving a false reading
What to check first
- Fix any misfire, lean or rich codes first: they damage converters and can cause this code
- Have exhaust leaks and the rear oxygen sensor checked before buying a converter, which is costly
- Be careful with "catalyst cleaner" claims: a worn converter cannot be restored by an additive
Have a mechanic confirm. A fault code tells you which system reported a problem, not which part has failed. The same code can have several causes, and they differ between car models. Use this page to understand the code, then have a qualified mechanic test the car before any part is replaced.
After the repair
Once the cause is fixed, the code can be cleared with a scan tool, or the warning light goes out by itself after several trips without the fault. Clearing a code without fixing the cause only hides it for a while. More about clearing codes.
Related codes
Exhaust gas recirculation (EGR) (P0400), Secondary air injection (P0410), Fuel vapour (EVAP) system (P0440), Diesel particulate filter (DPF) (P2002), Diesel exhaust fluid (AdBlue) and SCR system (P203F). All codes.
Written in November 2026. These are generic OBD-II codes, the same on all makes. Causes and repair steps differ between models.