What P0421 means
Catalyst 1 is the first catalyst in the flow direction of bank 1 (the side with cylinder 1) – the close-coupled or warm-up catalyst that lights off first after a cold start; the older wording was Warm Up Catalyst.
How the engine computer detects it
P0421 means it no longer converts enough. Mitsubishi V6 engines with manifold catalysts and GM's 3.0/3.2-liter V6 of 2004/05 with Bosch control report it: the rear sensor amplitude is compared with a model, and a quality factor above 0.625 (2004) or 0.375 (2005) sets the code after two trips. GM's small diesels use P0421 for the close-coupled oxidation catalyst, tested by heat balance during filter regeneration.
Written from public technical sources and not yet checked by a mechanic. How we write these pages
Symptoms
- Check engine light on, otherwise often no change in driving
- Emission test failed (higher HC, CO, NOx)
- Rattling or loss of power if the catalyst has broken up
Common causes
- Close-coupled catalyst aged, overheated or coating detached Most likely
- Misfire or rich running as prior damage Possible
- Sensor after the close-coupled catalyst sluggish or faulty Possible
- Engine module software (Mitsubishi: update for 2006 Eclipse V6) Less likely
How to diagnose P0421
- Read every stored code with an OBD-II scanner. If misfire or fuel trim codes are there too, fix those first: they often cause this one.
- Inspect the parts involved: close coupled catalyst, downstream oxygen sensor, engine control module.
- Work through the causes above from most to least likely, cheapest checks first.
- Clear the code and drive a full drive cycle to confirm the repair held.
Vehicles that report P0421 most
| Vehicle | Years | Reports | Share | Median miles |
|---|---|---|---|---|
| Mitsubishi Lancer | 2002–2003 | 4 | 98k |
Counts of owner complaints to the NHTSA that mention P0421 (11 in total), not failure rates: popular models get more complaints simply because more of them are on the road. Models with fewer than 3 complaints are left out.
Reading the code
- P
- Powertrain
- 0
- Generic (SAE) code
- 4
- Auxiliary emission controls
- 21
- Specific fault
Questions drivers ask
Can I drive with code P0421?
Usually yes for a short time, if the car drives normally, the temperature gauge reads normal and the check engine light is steady. If the light flashes, the engine loses power or overheats, or other warning lights come on, pull over and have it checked.
Will my car pass an emissions inspection with P0421?
Probably not in states that test vehicles with an OBD-II scan: an emissions-related code that keeps the check engine light on is usually an automatic fail. Rules differ by state and county, so check yours.
Will P0421 clear by itself?
The light can go off after several drive cycles without the fault, but the code returns if the cause is still there. After a repair, the readiness monitors need a full drive cycle before an inspection.
Can I fix P0421 myself?
Often yes, with an OBD-II scanner and basic tools. Difficulty: medium. Start with the cheapest checks before replacing expensive parts.
This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.