P0420
Catalyst System Efficiency Below Threshold (Bank 1)
What P0420 means
The downstream oxygen sensor is reporting a signal too similar to the upstream one. The computer concludes the catalytic converter is no longer doing enough work.
Symptoms you may notice
- Check engine light with no drivability change
- Failed emissions test
- Sometimes a sulfur smell
Common causes, cheapest first
- Genuinely failed catalytic converter
- Failing downstream oxygen sensor
- Exhaust leak ahead of or between the sensors
- A long-running misfire that damaged the converter
- Engine burning oil or coolant and poisoning the catalyst
What to do next
- Confirm the code with your own scan tool, or have it read free at a chain parts store.
- Decode your VIN so you know your exact engine and build date.
- Work down the causes above from cheapest to most expensive.
- Search NHTSA for a recall or service bulletin covering this symptom on your vehicle.
- Read the engine buying guide before ordering.
P0420 is a generic OBD-II code, standardized across every make and model sold in the US from 1996 onward. The meaning is the same on any vehicle; the specific part and its location are not.
Parts commonly needed for P0420
These go to Amazon. We earn a commission if you buy, at no extra cost to you — see our affiliate disclosure. Which one you actually need depends on diagnosis — read the note above first.
Confirm fitment against your VIN on the listing page before ordering. A part that matches your year and model can still be wrong for your build date or engine code.