P0421: Warm Up Catalyst Efficiency Below Threshold (Bank 1)
Key takeaways
- P0421 means the engine computer's test of the small warm-up converter closest to the engine on bank 1 failed; it does not by itself prove the converter is the bad part.
- If the car runs normally you can usually keep driving while you book a diagnosis, but a flashing light, shaking or a sudden loss of power changes that.
- Rule out an exhaust leak, an aging oxygen sensor, a misfire and any software bulletin first; those cost $100 to $800 against $1,000 to $2,500 for a converter.
- Most of the 57 manufacturer bulletins naming P0421 are software updates for heavy-duty diesel trucks; for gas cars, Mazda has a PCM logic fix for a false P0421 on some 2014–2016 models.
- The catalytic converter is covered by the federal emissions warranty for 8 years or 80,000 miles, so check the odometer before you pay.
What P0421 means
The small catalytic converter closest to the engine on bank 1 isn’t cleaning the exhaust as well as the engine computer expects.
Scanners call it “Warm Up Catalyst Efficiency Below Threshold (Bank 1).” A warm-up converter sits at or inside the exhaust manifold so it heats up quickly after a cold start. The computer judges it the same way it judges the main converter: it compares the oxygen sensor ahead of the converter, which swings rich and lean as the engine trims fuel, with the sensor behind it, which should stay fairly steady. When the rear signal starts copying the front one, the computer decides the converter has lost its ability to store oxygen and sets P0421. On diesel trucks, some makers use the same code for the oxidation catalyst in the exhaust aftertreatment system.
Bank 1 is the side of the engine with cylinder 1; the other side of a V6 or V8 sets P0431. The code means the test failed, not that the converter is proven bad.
Is it safe to drive?
Yes, in most cases. If the car starts, idles and pulls normally, P0421 on its own won’t strand you, and you can book a diagnosis in the next few weeks. You will fail an OBD emissions inspection while the light is on.
The answer changes if the light starts flashing, the engine shakes, or the car feels choked and gutless on hills. A flashing light is an active misfire overheating the converter right now; ease off and get off the road. A converter that has broken up inside can block the exhaust and kill power, and then it’s a tow. An exhaust smell in the cabin also needs a look soon, because it can mean a leak near the manifold.
Causes, ranked
| Cause | How you confirm it | Typical cost (US independent shop, 2026) |
|---|---|---|
| Exhaust leak near the manifold or ahead of the rear sensor | Ticking or puffing at cold start, soot at a joint, a clamp out of place, smoke test | $150–$600 |
| Engine software needs an update | Shop checks for a bulletin on your make, model and engine | $0–$200 |
| Aging oxygen sensor | Slow or odd readings in live data on a warm engine; sensor or heater codes | $150–$400 per sensor |
| Misfire, rich running or oil burning upstream | Other stored codes (P0300, fuel trim codes), high fuel trims, oil use | $150–$800 for the usual fix (plugs, coil, injector) |
| Warm-up converter worn out or its coating separated | Rear sensor mirrors front after the above are ruled out; rattle inside the manifold | $1,000–$2,500 per converter |
When the converter is built into the exhaust manifold, the part and labor sit toward the top of that range. Trucks and California-compliant converters can go above it.
What to check first
- Read every stored code, not just P0421. Misfire or fuel trim codes get fixed first; a converter fed raw fuel will fail again.
- Listen at a cold start for ticking or puffing near the manifold and front pipe. Leaks often seal up as the metal warms.
- Look at the rear oxygen sensor in live data on a fully warm engine at a steady cruise. A fairly flat signal means the converter is probably fine; one that switches like the front sensor points at the converter.
- Ask about bulletins and warranty. The bulletin pattern here is lopsided. Most cover diesel trucks: engine computer reprogramming for 2016 and 2019 Ram 2500/3500, plus 2020 Silverado and Sierra HD and Hino medium trucks. For gas cars, Mazda changed the engine computer logic to stop a false P0421 on some 2014–2016 Mazda3, Mazda6 and CX-5 models, Mitsubishi covers manifold-mounted converters that can separate internally on the 2006–2008 Eclipse and Galant, and GM has a bulletin to reposition a misinstalled exhaust clamp on the 2018 Equinox. The converter also carries a federal 8-year/80,000-mile emissions warranty.
- Clear it and drive only after a repair. Clearing resets the readiness monitors, and the catalyst test can take several days of mixed driving to run again.
Of the 9 owner complaints filed with NHTSA that mention P0421, 4 are for the Mitsubishi Lancer, model years 2002 and 2003. That’s a count of owner reports, not a confirmed defect.
What it costs
The cheap outcome is a software update or a repositioned clamp, from free under warranty to about $200, or an exhaust leak or oxygen sensor at $150–$600. The common outcome is a sensor or an upstream fix like plugs or a coil, up to about $800. The expensive outcome is the converter at $1,000–$2,500. A diagnosis at $100–$180 is what keeps you from buying a converter to fix a leak. If you want help reading your live data, ask our AI mechanic.
Related codes
P0421 often shows up with P0420 on the same bank, with P0431 on V engines, or with P0422, the main catalyst on bank 1. P0430 and oxygen sensor codes like P0138 point the same way. For what a steady light means in general, see check engine light on.
What owners report to NHTSA
9 complaints filed with NHTSA mention P0421, 5 of them since 2015, across 6 different models. The component owners most often filed it under: engine.
Most-reported models
| Vehicle | Complaints | Model years reported |
|---|---|---|
| MITSUBISHI LANCER | 4 | 2002, 2003 |
Manufacturer service bulletins that name P0421
57 technical service bulletins and manufacturer communications filed with NHTSA mention P0421, from 6 makes. A bulletin means the manufacturer has documented a known cause or a fix procedure for that model. Examples:
- HINO HINO, bulletin 10173038: SB-20-001 If a truck comes to the dealer with a MIL lamp illuminated with the Diagnostic Trouble Code (DTC): P0421 Determination of ATC Deterioration. Certain 16MY-20MYCab-Over Engine (COE) Trucks equipped with a J05 eng…
- CHEVROLET SILVERADO 2500 (2020), bulletin 10210030: This technical bulletin provides repair information to correct a customer concern of Service Engine Soon Lamp being illuminated with DTCs P2002, P2463, P2459, P144E, P0421, P0422, P0106 P20EE, P129F, P16F3, P20FF, P226D,…
- GMC SIERRA 3500 (2020), bulletin 10210030: This technical bulletin provides repair information to correct a customer concern of Service Engine Soon Lamp being illuminated with DTCs P2002, P2463, P2459, P144E, P0421, P0422, P0106 P20EE, P129F, P16F3, P20FF, P226D,…
- GMC SIERRA 2500 (2020), bulletin 10214212: This technical bulletin provides repair information to correct a customer concern of Service Engine Soon Lamp Illuminated, Harsh or Delayed Transmission Shifts Including Slip, Surge, Shudder, Noise, Whine, Growl, Shake a…
- RAM 2500 (2019), bulletin 10180257: Flash: Powertrain Control Module (PCM) Updates This bulletin involves reprogramming the Engine Control Module (ECM) / Powertrain Control Module (PCM) with the latest available software. Customers may experience a Malfunc…
- RAM 3500 (2019), bulletin 10220943: Flash: Powertrain Control Module (PCM) Updates This bulletin involves reprogramming the Engine Control Module (ECM) / Powertrain Control Module (PCM) with the latest available software. Customers may experience a Malfunc…
Sources: NHTSA Office of Defects Investigation complaint database and manufacturer communications file, snapshots September 2026. A complaint or bulletin that mentions a code is a report about that vehicle, not a statement that the code has one cause.
Frequently asked questions
What is a warm-up catalyst?
It's a small catalytic converter mounted right at the exhaust manifold, or built into it, so it heats up within seconds of a cold start. It does most of the cleaning while the engine warms. Not every car has one, which is why many cars never set P0421.
What is the difference between P0420 and P0421?
P0420 tests the main catalyst system on bank 1. P0421 tests the warm-up converter closest to the engine. Some makers use one, some the other, and the fixes overlap: exhaust leaks, oxygen sensors, misfires, then the converter.
Can I drive with a P0421 code?
Usually yes. If the engine idles smoothly, pulls normally and the light is steady, it's an efficiency warning. Get it diagnosed within a few weeks, and stop driving if the light starts flashing or the car suddenly loses power.
Is P0421 covered by warranty?
The converter often is. Federal rules require carmakers to cover the catalytic converter for 8 years or 80,000 miles. The repair has to go through a manufacturer-authorized shop, so call the dealer first if your car is inside that window.