Powertrain, generic · Auxiliary emission controls

P0421 Catalyst 1 Efficiency Below Threshold Bank 1

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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.

Likely causes

Close-coupled catalyst aged, overheated or coating detached
Misfire or rich running as prior damage
Sensor after the close-coupled catalyst sluggish or faulty
Engine module software (Mitsubishi: update for 2006 Eclipse V6)

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

Repair estimate

DifficultyMediumDIY possible
Parts and labour€250–2,000
Time1–3.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

close coupled catalystdownstream oxygen sensorengine control module

Related codes

P0420P0422P0431P0430P0300

References: static.nhtsa.gov, gsi.ext.gm.com, gsi.ext.gm.com, gsi.ext.gm.com.