Engine and gearbox (P)
P0420: Catalyst efficiency below threshold
The catalytic converter is not working efficiently enough to reduce emissions as expected. This can lead to increased pollution and may affect engine performance.
Checked by a licensed car technician · last reviewed 29 September 2026
Symptoms
- Check engine light is on
- Increased emissions
- Possible decrease in fuel efficiency
- Rattling noise from under the car
Common causes
- Failing catalytic converter
- Upstream O2 sensor drift
- Exhaust leak
- Downstream O2 sensor fault
- Engine misfire
How to check it yourself
- Inspect for exhaust leaks
- Check the operation of the upstream O2 sensor
- Examine the catalytic converter for damage
- Test the downstream O2 sensor
- Check for engine misfires
Can I keep driving?
The car can usually be driven, but it should be repaired soon to avoid increased emissions and potential damage to the catalytic converter.
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