C0021
5 owner reports and 21 manufacturer filings across 11 model years from Cadillac, Chevrolet, GMC, Mazda.
Where C0021 shows up
Source: NHTSA manufacturer communications and complaints| Model year | Filed under | Bulletins | Owner reports | Parts |
|---|---|---|---|---|
| 2019 Chevrolet Silverado | Service Brakes | 6 | 4 | Brake pads → |
| 2019 GMC Sierra | Electrical System | 6 | 1 | Battery → |
| 2020 Cadillac CT6 | Service Brakes | 1 | — | Brake pads → |
| 2019 Cadillac CT6 | Service Brakes | 1 | — | Brake pads → |
| 2019 Cadillac XT5 | Service Brakes | 1 | — | Brake pads → |
| 2019 GMC Acadia | Service Brakes | 1 | — | Brake pads → |
| 2021 Mazda CX-30 | Service Brakes | 1 | — | Brake pads → |
| 2020 Mazda CX-30 | Service Brakes | 1 | — | Brake pads → |
| 2021 Mazda Mazda3 | Service Brakes | 1 | — | Brake pads → |
| 2020 Mazda Mazda3 | Service Brakes | 1 | — | Brake pads → |
| 2019 Mazda Mazda3 | Service Brakes | 1 | — | Brake pads → |
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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C0021 by make
The same record, one make at a time: the model years, bulletins and owner reports that name C0021 on that maker’s vehicles.
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.
Brakes and steering limited could not drive unsafe turned off ignition restarted threw five codes including P0606 C0021 P25A2 called onstar, they said all systems showing failed take to dealer called gm- they said to have it towed remote…
21 manufacturer filings name C0021. 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 C0021.
- SA-044-21 Service bulletin2021-07-13 · Service Brakes, Hydraulic
- When switching to the maintenance mode, U3000:53 and the related DTCs are stored, but the DTCs will not be cleared automatically after finishing the maintenance mode. It is necessary to clear the DTCs completely after performing the brake system maintenance mode. DTC codes: U3000:53, C0021:52, U0415:68
- 20NA073 Service bulletin2020-09-22 · Service Brakes
- This technical bulletin provides repair information to correct a customer concern of Loss of Brake Assist, Various Messages Displayed in Driver Information Center (DIC), Brake Pedal Travel and Apply May Feel Different, One or More of the Following DTCs May be Set: P0606 with C0021, C05D2, C0595, C2A3B, C0024, C053D, P25A2.
- PIT5728B Service bulletin2020-01-15 · Electrical System
- This Preliminary information communicates an issue with DTC P0606 and C0021 setting after completing campaign N192268090 and advises to reprogram the BSCM with updated calibrations available in Tis2Web.
- PIT5728A Service bulletin2020-01-06 · Engine And Engine Cooling
- This Preliminary information communicates an issue with DTC P0606 and C0021 setting after completing campaign N192268090 and it provides information about using and maintaining proper battery voltage while programming the BSCM.
- PIT5728 Service bulletin2019-12-20 · Electrical System
- This Preliminary information communicates an issue with DTC P0606 and C0021 setting after completing campaign N192268090 and it provides information about using and maintaining proper battery voltage while programming the BSCM.
What C0021 means
Source: SAE J2012 / OBD-IIC0021 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 C0021
Source: NHTSA files and the OBD-II standardCan I keep driving with C0021 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.