P0421: Catalyst 1 Efficiency Below Threshold Bank 1

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.

Medium severity. Can I keep driving?You can usually keep driving, unless you notice a loss of power, which may point to a blockage. The car may not pass an emissions inspection.

Bank 1 is the side of the engine with cylinder 1. Inline engines only have this bank.

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

Most 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)

What to check, in order

Start with the simplest. You can tick off each step.

First resolve any misfire or fuel mixture codes
Look for exhaust leaks between the engine and the second sensor
Use live data to compare the signals from both oxygen sensors
Check for a blockage by measuring exhaust back pressure
Replace the catalytic converter only at the end of the diagnosis

Good to know

Repair difficulty: medium
It affects emissions.

What each character of P0421 means

P: Powertrain: engine and transmission
0: Generic code: same on every make
4: Emissions
21: Specific fault in that system

Frequently asked questions

Can I keep driving with code P0421?

You can usually keep driving, unless you notice a loss of power, which may point to a blockage. The car may not pass an emissions inspection.

What causes code P0421?

The most common causes are: close-coupled catalyst aged, overheated or coating detached; misfire or rich running as prior damage; sensor after the close-coupled catalyst sluggish or faulty.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.