U0121: owner reports and bulletins
Got U0121 on your scanner? Tell the mechanic your year, make, model and any symptoms and you'll get the likely cause and what to check first. Ask about this code.
What owners report to NHTSA
39 complaints filed with NHTSA mention U0121, 34 of them since 2015, across 28 different models. The component owners most often filed it under: service brakes.
Most-reported models
| Vehicle | Complaints | Model years reported |
|---|---|---|
| CHEVROLET MALIBU | 6 | 2009, 2010, 2014, 2020 |
| FORD F-150 | 3 | 2017, 2020, 2021 |
Manufacturer service bulletins that name U0121
76 technical service bulletins and manufacturer communications filed with NHTSA mention U0121, from 8 makes. A bulletin means the manufacturer has documented a known cause or a fix procedure for that model. Examples:
- FORD F-150 (2024), bulletin 10254172: Some 2024 F-150 non-Lightning vehicles equipped with minor feature codes C2BAW and FDDAA may exhibit an intermittent Trailer Brake System Fault See Manual message in the instrument panel cluster (IPC), inoperative traile…
- JEEP WRANGLER (2018, 2019, 2020, 2021), bulletin 10222946: ABS Lamp On, U0121 Lost Communication With ABS Module, Stop Start Inoperative U0037-00 LIN BUS
- RAM 1500 (2021), bulletin 10190705: Flash: Hybrid Control Processor (HCP) Updates This bulletin involves updating the HCP with the latest available software. Customers may experience a Malfunction Indicator Lamp (MIL) illumination. Upon further investigati…
- FORD MUSTANG MACH E (2021), bulletin 10203653: Some 2021 Mustang Mach-E vehicles built on or before 09-Sep-2021 may exhibit an illuminated powertrain malfunction indicator (wrench) lamp and/or display the message Shift System Fault Service required with DTC U0103:8…
- CHEVROLET SILVERADO 1500 (2019, 2020), bulletin 10176304: 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 mess…
- GMC SIERRA 1500 (2019, 2020), bulletin 10176304: 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 mess…
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.