P0421: Catalyst 1 Efficiency Below Threshold, Bank 1
P0421 means the first catalytic converter on bank 1 (the side of the engine with cylinder 1) is no longer cleaning exhaust well enough. The car usually drives normally, but the check engine light stays on and the car can fail an emissions test.
You can drive, but get it checked soon
The code does not normally change how the car drives and does not trigger limp mode, so short-term driving is usually possible. A misfire or a broken-up catalyst can cause damage or power loss, so have it checked soon, especially if the car rattles, loses power or the light flashes.
What it means
Catalyst 1 is the first catalytic converter in the exhaust flow on the bank 1 side. It sits close to the engine, so it heats up first after a cold start. Older documents call it the Warm Up Catalyst. The engine computer compares the signal of the sensor behind the catalyst with a model of a healthy one. If the catalyst converts too little, the code is stored, and the light comes on after the fault shows up on two trips. Some Mitsubishi V6 engines with manifold catalysts and GM 3.0 and 3.2-liter V6 engines from 2004 and 2005 report this code this way. On small GM diesels, P0421 refers to the close-coupled oxidation catalyst, which is checked by comparing heat during filter regeneration.
Likely causes
- Close-coupled catalyst aged, overheated or coating detachedVery likely
The catalyst's coating wears out or breaks away over time or from heat, so it converts less of the exhaust gases.
- Misfire or rich running as prior damagePossible
Unburned fuel or an overly rich mixture can overheat and damage the catalyst, so the cause may be an engine problem that came first.
- Sensor after the close-coupled catalyst sluggish or faultyPossible
The computer judges the catalyst from this sensor's signal, so a slow or faulty sensor can make a good catalyst look bad.
- Engine module softwareLess likely
For Mitsubishi, an update exists for the 2006 Eclipse V6, so the evaluation itself can be the issue in rare cases.
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
What to check, cheapest first
- 1Read all stored codes and freeze-frame data. Look for misfire codes such as P0300 and for related codes like P0420, P0422, P0430 or P0431. · you can do it
- 2Look for exhaust leaks near the manifold and catalyst, and listen for rattling from the catalyst when tapping it lightly once the exhaust is cold. · you can do it
- 3Fix any misfire or rich-running problem first, since it can damage a new catalyst too. · mechanic
- 4Check the sensor behind the close-coupled catalyst with a scan tool, including its wiring and response. · mechanic
- 5Ask whether a software update exists for the engine module on your model, for example the one for the 2006 Mitsubishi Eclipse V6. · mechanic
- 6If the catalyst is confirmed worn, replace it, then run the drive cycle so the monitor can complete and confirm the repair. · mechanic
Related codes
Frequently asked questions
Can I drive with a P0421 code?+
Usually yes for a short time, since driving often feels normal. Get it checked soon, and stop sooner if you notice rattling, power loss or a flashing light.
Can a loose gas cap cause P0421?+
It is not a listed cause of this code. A loose cap typically sets evaporative system codes instead, so check the cap but do not expect it to clear P0421.
Will a car with P0421 pass an emissions test?+
Probably not. The code is emissions-relevant and the light is on, and the worn catalyst can raise hydrocarbon, carbon monoxide and nitrogen oxide readings.
How long does the repair take?+
The reference estimates about 1 to 3.5 hours of work, and a DIY repair is possible depending on the cause.
Written with the help of AI from the public OBDex definition (CC0). Each make may use the code differently. Updated: 2026-11-10