P0431: Catalyst 1 Efficiency Below Threshold, Bank 2
P0431 means the first catalytic converter on bank 2 (the side of the engine without cylinder 1) is no longer cleaning the 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 data lists no limp mode for this code and driving often feels unchanged, so short-term driving is usually possible. If the catalyst has broken up and you hear rattling or notice power loss, or if a misfire is also present, get it checked soon because those problems can do more damage.
What it means
The catalytic converter (catalyst) is the exhaust part that turns harmful gases into less harmful ones. Catalyst 1 on bank 2 is the first one in the exhaust flow on that side, the one close to the engine that heats up first after a cold start. Older documents call it the Warm Up Catalyst. The engine computer compares the signal of the oxygen sensor behind the catalyst with a model of how a healthy catalyst should behave. When the signal shows the catalyst is converting too little, the computer sets P0431 and turns on the check engine light. In the data for this code, Mitsubishi V6 engines with manifold catalysts and GM's 3.0/3.2-liter V6 of 2004/05 with Bosch control are named as reporting it, and the code is set after two trips (two separate drives in which the test runs). A Mitsubishi bulletin names coating coming off the manifold catalysts as one cause.
Likely causes
- Close-coupled catalyst aged, overheated or coating detachedVery likely
This is the most common reason, since the catalyst wears out or its coating comes loose, so it can no longer convert enough exhaust gas.
- Misfire or rich running as prior damagePossible
Unburned fuel or an overly rich mixture can overheat and damage the catalyst, so the code may be the result of an earlier problem.
- Sensor after the close-coupled catalyst sluggish or faultyPossible
The computer judges the catalyst by this sensor's signal, so a slow or faulty sensor can make a working catalyst look bad.
- Engine module software (Mitsubishi: update for 2006 Eclipse V6)Less likely
The evaluation logic can be corrected by a software update, which Mitsubishi lists for the 2006 Eclipse V6.
What you may notice
- Check engine light on, otherwise often no change in driving
- Failed emissions test (higher HC, CO and NOx readings)
- Rattling or loss of power if the catalyst has broken up
What to check, cheapest first
- 1Read the stored codes and look for related ones such as P0430, P0432, P0421, P0420 or P0300 (misfire). Fix misfire codes first. · you can do it
- 2Listen for rattling from the exhaust when the engine is cold and look for exhaust leaks near the manifold and catalyst on bank 2. · you can do it
- 3Check the service history and engine for signs of misfire or rich running, such as rough idle or fuel smell, which can damage the catalyst. · you can do it
- 4Look at live data of the oxygen sensor behind the catalyst and judge whether it responds quickly or is sluggish. · mechanic
- 5Check whether a software update for the engine module exists for your vehicle, as listed for the 2006 Mitsubishi Eclipse V6. · mechanic
- 6If the sensor and engine are fine, replace the bank 2 close-coupled catalyst, then clear the code and complete a drive cycle to confirm the repair. · mechanic
Related codes
Frequently asked questions
Can I keep driving with P0431?+
Usually yes for a short time, since the car often drives normally. Have it checked soon, and sooner if you hear rattling, lose power or the engine misfires.
Can a loose gas cap cause P0431?+
The data does not list the gas cap as a cause. P0431 points to the catalyst, a sensor behind it, prior misfire damage or software.
Will the car pass an emissions test with P0431?+
Probably not. The check engine light is on and the code is emissions-relevant, and a failing catalyst can show higher HC, CO and NOx readings.
How hard is the repair?+
The data rates it medium difficulty, with an estimated 1 to 3.5 hours, and a DIY repair is possible.
Written with the help of AI from the public OBDex definition (CC0). Each make may use the code differently. Updated: 2026-11-10