P0421: How to Diagnose & Fix — Warm Up Catalyst Efficiency Below Threshold (Bank 1)
6 vehicles · 5 makes · $150-$3,500 repair
How to diagnose and fix P0421: most-reported vehicles
Ranked by owner reports. Each card gives the symptoms, how to confirm the cause, the fix, parts and sources for that vehicle.
Mazda RX-8 · 2004–2012
The RX-8 catalytic converter clogs prematurely due to the rotary engine's naturally higher oil and fuel consumption. Unburned fuel and oil residue accumulate in the catalyst substrate, restricting exhaust flow. A clogged cat causes power loss, overheating, and can destroy the engine if backpressure becomes severe. Catalytic converter failure is often a secondary symptom of apex seal wear.
Common Symptoms
- Significant power loss at higher RPM
- Exhaust smells like sulfur (rotten eggs)
- Engine overheating due to restricted exhaust flow
- Check engine light with P0420/P0421 catalyst efficiency codes
- Glowing catalytic converter visible at night (extreme cases)
How to Fix
Replace catalytic converter. If apex seals are worn (causing excess oil burning), rebuild the engine first or the new cat will clog again. Some owners use high-flow aftermarket cats or test pipes for track use. Ensure O2 sensors are replaced at the same time if aged.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- MagnaFlow 49150 OEM-grade Federal/EPA direct-fit catalytic converterOEMMagnaFlow 49150
Federal-emissions direct-fit converter after restriction/failure and root cause are confirmed Fitment/availability: The inspected direct product page identifies part 49150 as specific direct-fit for 2004-2011 Mazda RX-8, interchanging wi…
As an Amazon Associate, we earn from qualifying purchases; other part links may also earn au7o a commission. Confirm fitment by VIN before buying.
Owner tips & cautions
- TipAddress apex seal wear FIRST - a worn engine will destroy a new cat quickly
- TipPremixing 2-stroke oil reduces unburned fuel in exhaust, extending cat life - RX8Club.com exhaust FAQ
Sources for this issue (2)
Mitsubishi Endeavor · 2004–2011
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.
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)
Mazda MPV · 2000–2006
Second-generation MPVs carry three catalytic converters: a main converter plus two close-coupled 'pre-cats' bolted right at the exhaust manifolds, one on the radiator side and one on the firewall side. Sitting that close to the ports, the pre-cat substrates run extremely hot and break down. The immediate symptom is a catalyst efficiency code - P0420 on earlier vehicles, P0421 (warm-up catalyst, bank 1) on Mazda's later coding - but the real hazard is that a disintegrating pre-cat can shed ceramic substrate which, on an engine that has any intake pulsation or a leaking exhaust valve, can be drawn back toward the cylinder and score the bore. The failure is almost always downstream of something else: a persistent misfire, a leaking injector, or an oil-consumption problem cooking the catalyst. Owners commonly replace the rear O2 sensor first and get the code back within a drive cycle.
Common Symptoms
- Check engine light with catalyst efficiency code
- Rotten-egg / sulfur smell from the exhaust
- Reduced power and poor fuel economy
- Rattle from the exhaust near the engine
- Failed emissions test
How to Fix
Do not replace the converter until the cause is found. Check for misfires and pending misfire counters, verify fuel trims, confirm the upstream air/fuel ratio sensor and downstream O2 sensor are switching correctly, and fix any oil consumption or coolant loss into a cylinder first - all of these destroy a replacement converter within months. Confirm the code by comparing the pre- and post-catalyst sensor traces rather than by code alone. When the converter genuinely is the fault, replace the failed pre-cat with a properly rated unit; a cheap universal converter will re-set the code. Budget for the fact that the pre-cats are hard to access on a transverse V6.
Sources for this issue (3)
Nissan Armada · 2004–2015
Toyota Prius · 2004–2025
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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?
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.
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?
Repair costs for P0421 range from $150 to $3,500, depending on the vehicle and root cause.
Which vehicles are affected by P0421?
Au7o has documented P0421 across 6 vehicle models from 5 manufacturers: Mazda, Mitsubishi, Nissan, Subaru, Toyota.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.