P0421Catalyst 1 Efficiency Below Threshold Bank 1
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- 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. P0421 means it no longer converts enough. Mitsubishi V6 engines with manifold catalysts and GM's 3.0/3.2-litre 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.
- What you may notice
- 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
- Likely causes
- LikelyClose-coupled catalyst aged, overheated or coating detached
- PossibleMisfire or rich running as prior damage
- PossibleSensor after the close-coupled catalyst sluggish or faulty
- RareEngine module software (Mitsubishi: update for 2006 Eclipse V6)
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Often possible to do yourself.Typical cost: €250 – €2,000
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.