U0121 A module has stopped hearing from the anti-lock brake controller
4 owner reports and 83 manufacturer filings across 26 model years from Cadillac, Chevrolet, Chrysler, Ford and others.
Where U0121 shows up
Source: NHTSA manufacturer communications and complaintsShowing the 25 model years where U0121 appears most; it is on file for 26 in total.
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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U0121 by make
The same record, one make at a time: the model years, bulletins and owner reports that name U0121 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.
Codes present are U0121 (lost communication with ABS control module A), U1012 (invalid internal control module data received from ABS control module), and U3035 (communication authentication signal invalid data)
83 manufacturer filings name U0121. 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 U0121.
- S2523000041 Service bulletin2025-05-19 · Electrical System
- Vehicle Will Not Shift From Park. Antilock Brake System (ABS) Light On In Cluster. U0121, U1110, U120C Diagnostic Trouble Codes (DTCs) Set. Discussion: If a vehicle has a complaint of will not shift from park and the below DTCs are set, follow the steps below prior to making any repairs. U0121-00-LOST COMMUNICATION WITH ANTI-LOCK BRAKE SYSTEM (ABS) U1110-LOST VEHICLE SPEED MESSAGE U120C-LOST BRAKE
- PIT5713A Service bulletin2020-12-22 · Lane Departure
- This Preliminary information communicates an issue with the engine wiring harness shorting at the left front cylinder head causing any of the following issues: - IPC gauges sweeping while driving - Several IPC warning messages and MIL displayed - Any combination of the following DTC's: U0073 U0100 U0101 U0121 U0128 U0129 U0131 U0140 U0146 U0422 U2413 P0107 P0117 P0119 P0641 C2A07
- 19NA068 Service bulletin2020-07-15 · Equipment
- This technical bulletin provides repair information to correct a customer concern of Malfunction Indicator Lamp (MIL) illuminated, intermittent service messages, Erratic Gauges and technician finding DTCs U0073, U0121, U0100 and/or U0101 set.
- PIT5713A Service bulletin2020-05-07 · Engine And Engine Cooling
- This Preliminary information communicates an issue with the engine wiring harness shorting at the left front cylinder head causing any of the following issues: - IPC gauges sweeping while driving - Several IPC warning messages and MIL displayed - Any combination of the following DTC's: U0073 U0100 U0101 U0121 U0128 U0129 U0131 U0140 U0146 U0422 U2413 P0107 P0117 P0119 P0641 C2A07
- 19NA068 Service bulletin2019-11-12 · Electrical System
- This technical bulletin provides repair information to correct a customer concern of Malfunction Indicator Lamp (MIL) illuminated, intermittent service messages, Erratic Gauges and technician finding DTCs U0073, U0121, U0100 and/or U0101 set.
- PIT5713 Service bulletin2019-09-30 · Engine And Engine Cooling
- This Preliminary information communicates an issue with the engine wiring harness shorting at the left front cylinder head causing any of the following issues: - IPC gauges sweeping while driving - Several IPC warning messages and MIL displayed - Any combination of the following DTC's: U0073 U0100 u0101 u0121 u0128 U0129 u0131 U0140 U0146 U0422 U2413 P0107 P0641 C2A07
What U0121 means
Source: SAE J2012 / OBD-IIA module has stopped hearing from the anti-lock brake controller.
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 U0121
Source: NHTSA files and the OBD-II standardCan I keep driving with U0121 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.