Fault-Code Library · Emissions
P0420: Catalyst efficiency below threshold, bank 1
Upstream and downstream oxygen-sensor behavior suggests insufficient catalyst oxygen-storage performance.
Common symptoms
- •Engine warning light
- •Possible sulfur smell
- •Usually little drivability change
Likely causes
- •Aged or damaged catalyst
- •Long-term misfire or mixture fault
- •Exhaust leak
- •Biased oxygen sensor
First checks
- •Repair mixture and misfire faults first
- •Check for exhaust leaks
- •Review live upstream/downstream sensor behavior
Important
Replacing the catalyst without correcting the cause can lead to another failure.
A fault code identifies a detected condition, not a failed part. Confirm manufacturer-specific subcodes and live data.