P0421 on Mitsubishi: How to Diagnose & Fix
Warm Up Catalyst Efficiency Below Threshold (Bank 1)
1 Mitsubishi model · 1 documented issue · $600-$2,000 repair
P0421 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)
P0421 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 P0421 mean on a Mitsubishi?
P0421 stands for "Warm Up Catalyst Efficiency Below Threshold (Bank 1)." The warm-up catalyst is a small pre-catalytic converter mounted close to the engine so it heats up fast and cleans exhaust during the first several minutes after a cold start, before the main converter reaches operating temperature. The engine computer compares the upstream and downstream oxygen sensor signals during this warm-up window; if the downstream sensor's readings look too similar to the upstream sensor's, the bank 1 catalyst (the side with cylinder #1) is no longer storing and converting emissions efficiently, and P0421 sets with a check engine light. To the driver it usually feels like nothing at all — perhaps slightly worse fuel economy, a faint rotten-egg smell, or in rare cases mild hesitation — but the vehicle will typically fail an emissions test. It is safe to keep driving short-term, but ignoring it allows the underlying cause (often a misfire, fuel-mixture fault, or oil/coolant contamination) to finish off the catalyst, and a disintegrating pre-cat can create exhaust restriction that hurts performance and, in extreme cases, the engine itself. On Mitsubishi it is documented across 1 model: Endeavor.
What are the most common causes of P0421?
The most common causes of P0421 are: Worn or degraded warm-up (pre-cat) catalytic converter on bank 1, Faulty downstream (post-cat) oxygen sensor giving inaccurate readings, Exhaust leak at or upstream of the warm-up catalyst skewing sensor data, Engine misfires or an overly rich/lean fuel mixture damaging the catalyst, Damaged or corroded oxygen sensor wiring/connectors, Oil or coolant contamination of the catalyst (worn valve seals, head gasket leak), Faulty engine coolant temperature sensor distorting the warm-up monitor. The specific cause varies by vehicle.
How much does it cost to fix P0421 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 P0421 on a different make?
View P0421 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.