P0431 on Mitsubishi: How to Diagnose & Fix
Warm Up Catalyst Efficiency Below Threshold (Bank 2)
1 Mitsubishi model · 1 documented issue · $600-$2,000 repair
P0431 on Mitsubishi by model: symptoms, diagnosis and fix
The 6G75 Endeavor runs three catalysts: a pre-cat integrated at each exhaust manifold plus an unmonitored underfloor unit. Only the two manifold pre-cats are monitored, so a check-engine light for catalyst efficiency always points at a manifold cat, never at the big underfloor converter that most quick-lube shops quote. Owners and techs confirm the mapping on this engine: the right (windshield-side) bank sets P0421 and the left (radiator-side) bank sets P0431. The pre-cats sit right at the head and cook, and they are commonly killed early by an upstream problem - a misfire or leaking injector dumping raw fuel into the converter.
Common Symptoms
- Check engine light with catalyst efficiency code
- Failed emissions/OBD test
- Sulfur or rotten-egg smell from the exhaust
- Reduced fuel economy
- Rattle from the manifold area on a broken substrate
How to Fix
Fix the cause before the converter. Verify no misfire codes, no fuel trim fault, and no exhaust leak upstream of the rear O2 sensor, and confirm the rear O2 sensor itself is switching correctly - a lazy rear sensor sets these codes with a healthy cat. If the pre-cat is genuinely spent, the repair is the manifold assembly for the affected bank; use the P0421 = right bank / P0431 = left bank mapping to avoid replacing the wrong side. Replacing the underfloor converter will not clear either code.
Sources for this issue (2)
P0431 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0431 mean on a Mitsubishi?
P0431 stands for "Warm Up Catalyst Efficiency Below Threshold (Bank 2)." On vehicles with a primary/warm-up catalyst, the PCM determines the Bank 2 warm-up catalyst (WU-TWC) is below its efficiency threshold, based on the upstream/downstream O2 sensor comparison for that bank. On Mitsubishi it is documented across 1 model: Endeavor.
What are the most common causes of P0431?
The most common causes of P0431 are: Worn/failed Bank 2 warm-up catalytic converter, Aged or contaminated O2 sensor on Bank 2, Exhaust leak near the warm-up catalyst, Misfire or improper air/fuel on Bank 2, Oil/coolant contamination. The specific cause varies by vehicle.
How much does it cost to fix P0431 on a Mitsubishi?
Published repair estimates on Mitsubishi range from $600 to $2,000 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
Looking for P0431 on a different make?
View P0431 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.