What does P0420 mean?
The control module detected a condition described as “Catalyst-system efficiency below threshold bank 1”. The code identifies the affected area, but it does not by itself prove which component has failed. The code describes the detected condition, not a guaranteed failed part. A reliable diagnosis also considers related codes, freeze-frame data, live values and the actual symptoms.
Possible causes
- Aged or heat-damaged catalytic converter
- Exhaust leak between the oxygen sensors
- Faulty oxygen sensor
- Long-standing mixture or misfire problem
Common symptoms
- Check-engine light
- Sometimes no noticeable driving change
- Higher fuel consumption or exhaust smell
- Power loss if damage is severe
Sensible next checks
- Rule out fuel-mixture and misfire faults first
- Compare upstream and downstream oxygen-sensor signals
- Inspect the exhaust system for leaks
- Do not replace the catalyst based on this code alone
Why does it return after clearing?
Clearing the code only removes the stored entry. If the underlying condition remains, the control module will set the code again after the next relevant monitor cycle. Save the data first, diagnose the cause, repair it and only then clear the code.