P0421
2 owner reports and 43 manufacturer filings across 9 model years from Chevrolet, GMC, Mazda, Ram.
Where P0421 shows up
Source: NHTSA manufacturer communications and complaints| Model year | Filed under | Bulletins | Owner reports | Parts |
|---|---|---|---|---|
| 2016 Ram 3500 | Engine | 14 | — | |
| 2016 Ram 2500 | Power Train | 11 | 1 | |
| 2020 Chevrolet Silverado 2500HD | Power Train | 4 | — | |
| 2020 GMC Sierra 2500HD | Power Train | 4 | — | |
| 2020 Chevrolet Silverado 3500HD | Engine | 3 | — | |
| 2020 GMC Sierra 3500HD | Engine | 3 | — | |
| 2017 Chevrolet Cruze | Engine | 2 | — | |
| 2016 Mazda Mazda6 | Engine | 2 | — | |
| 2020 Mazda CX-30 | Engine | — | 1 |
A model year appears here because the code is named in its record, not because every one of these vehicles sets it. Counts are mentions in the federal files, not failure rates — nobody publishes the denominator.
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What it gets filed under
What owners said
Source: NHTSA complaint databaseOwners’ own words, one per nameplate, scrubbed of anything identifying. These are unverified reports filed with NHTSA — they are what was said, not a finding that it was true.
It is making weird noises while shifting and is now throwing a CEL with the code P0421
43 manufacturer filings name P0421. Start with what the maker wrote down.
A code names the test that failed, not the part. A bulletin that names it is often a diagnosis someone has already written down — quote its number at the service desk.
Manufacturer filings that name it
Source: NHTSA manufacturer communicationsService bulletins and campaigns whose filed summary names P0421.
- 21-NA-214 Service bulletin2023-08-10 · Electrical System
- This technical bulletin provides repair information to correct a customer concern of Service Engine Soon Lamp being illuminated. They may also comment on multiple transmission issues including harsh or delayed shifts, slip, surge, shudder, noise, whine, growl, shake and/or vibration. As well as the exhaust brake not working or that they do not feel it. Technicians may find DTCs P2002, P2463, P2459
- 21-NA-214 Service bulletin2022-04-01 · Power Train
- 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, P249D, P249E, P11CC and / or P11D5. They may also comment on multiple transmission issues including harsh or delayed shifts, slip, surge, shudder, noise, whine, growl, shake and/o
- 21NA214 Service bulletin2022-02-16 · Power Train
- This technical bulletin provides repair information to correct a customer concern of Service Engine Soon Lamp being illuminated. They may also comment on multiple transmission issues including harsh or delayed shifts, slip, surge, shudder, noise, whine, growl, shake and/or vibration. As well as the exhaust brake not working or that they do not feel it. Technicians may find DTCs P2002, P2463, P2459
- 21NA214 Service bulletin2022-01-27 · Engine And Engine Cooling
- 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 and/or Vibration, DTCs P2002, P2463, P2459, P144E, P0421, P0422, P0106 and/or P20EE Set Current or in Recent History.
- PIE0420A Service bulletin2017-10-04 · Engine
- This engineering informational bulletin is requesting information from the field on vehicles that have the condition of Malfunction Indicator Lamp Illuminated and DTC P0421 Set.
- PIE-0420A Service bulletin2017-10-01 · Equipment — Electrical
- Engineering Information Malfunction Indicator Lamp Illuminated and DTC P0421 Set
What P0421 means
Source: SAE J2012 / OBD-IIP0421 sits in the generic range of the OBD-II standard, so it means the same thing on every vehicle sold in the United States — but WhichTrim does not carry a definition for it and will not invent one. What is on this page is the record: who named it and on what.
A stored code names the test that failed, never the part that caused it. It is the starting point of a diagnosis, not its conclusion — the same code has several possible causes and the cheap one is not always right.
Common questions about P0421
Source: NHTSA files and the OBD-II standardCan I keep driving with P0421 stored?
Nobody can answer that from the code alone, and a site that does is guessing. A steady light means a fault was logged and the car should be checked soon; a flashing light means a severe misfire — slow down and get it seen as soon as possible, because severe misfire can quickly damage the catalytic converter. The safe read of this page is the mileage and the components, not permission.
Does a code mean the car is recalled?
No. Recalls are filed by campaign and matched by VIN, and none of them are triggered by a scan tool. If you want to know whether an open recall applies to one specific vehicle, check the VIN at nhtsa.gov/recalls — that is the only source that settles it.
Why do the counts here differ from other sites?
Because these are not estimates. Every number on this page is a count of documents in NHTSA's own files: manufacturer communications that name the code in their filed summary, and owner complaints that name it in the narrative. We do not model, weight or extrapolate them.
Will clearing the code fix it?
Clearing turns the light off and erases the freeze-frame data a technician would have used to work out what happened. If the condition is still there the code returns, usually within a few drive cycles, and the diagnosis now starts from less information than before.