P0421 on Mitsubishi
Warm Up Catalyst Efficiency Below Threshold (Bank 1)
P0421 on Mitsubishi vehicles indicates warm up catalyst efficiency below threshold (bank 1). Au7o has documented this code across 1 Mitsubishi model — most commonly on Endeavor. 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. Typical repair costs on Mitsubishi range from $600 to $2,000, depending on the specific model and root cause.
Common Causes of P0421
- •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
P0421 on Mitsubishi by Model
Mitsubishi Endeavor(1 issue)
- Manifold Pre-Catalyst Efficiency Failure Setting P0421 or P04312004-2011
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.
Looking for P0421 on a different make?
View P0421 across all makes →Frequently Asked Questions
What does P0421 mean on 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 specifically, this code is documented across 1 model.
What causes P0421 on Mitsubishi vehicles?▼
Common causes on Mitsubishi: 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. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix P0421 on a Mitsubishi?▼
Repair costs on Mitsubishi range from $600 to $2,000, depending on the specific model and root cause.
Which Mitsubishi models have P0421 documented?▼
Au7o has documented P0421 on 1 Mitsubishi model: Endeavor.