Engine and gearbox (P)
P0421: Warm-up catalyst efficiency low
The catalytic converter is not performing efficiently during the engine's warm-up phase. This can lead to increased emissions.
Checked by a licensed car technician · last reviewed 29 September 2026
Symptoms
- Increased emissions
- Possible decrease in fuel efficiency
- Check engine light on
Common causes
- Catalytic converter ageing
- Oxygen sensor drift
- Exhaust leak
- Faulty coolant temperature sensor
How to check it yourself
- Check coolant temperature sensor readings
- Inspect for exhaust leaks
- Test oxygen sensor response
- Examine catalytic converter for damage
Can I keep driving?
The car can still be driven, but emissions may be higher than normal.
General guidance, not a professional inspection. Confirm faults with measurements before buying parts, and use a qualified mechanic for brakes, steering, fuel leaks, airbags, and other safety-critical work. Terms and disclaimer