Network communication (U-codes) · Network · generic (SAE-defined)
U0121: Lost Communication With Anti-Lock Brake System (ABS) Control Module
U0121 means other control modules stopped receiving network messages from the anti-lock brake system (ABS) control module.
Standard title per SAE J2012 / ISO 15031-6 · Updated 2026-11-07 · By CarWhere Research
How U0121 sets
The ABS module shares wheel speed and brake data over the controller area network (CAN). When the engine control module or another module stops receiving those messages for longer than allowed, it stores U0121. Timing and conditions vary by manufacturer.
Common causes of U0121
Roughly most likely first; the actual cause depends on the vehicle and needs testing to confirm.
- Blown ABS module fuse
- Corroded or loose ABS module connector
- Poor ABS module ground
- Damaged CAN wiring to the ABS module
- Low battery voltage
- Failed ABS control module
U0121 symptoms
- ABS and traction control warning lights on
- Check engine light on in some vehicles
- Speedometer or cruise control not working
- Stability control disabled
- Transmission shifting oddly due to missing speed data
How U0121 is diagnosed
- Scan all modules and confirm whether the ABS module communicates directly with the scan tool.
- Check ABS fuses, including high-current pump motor fuses.
- Test battery voltage and the ABS module ground.
- Inspect the ABS connector, which is often near the wheel well and exposed to water and salt.
- Check CAN wiring continuity between the ABS module and the network.
- Replace or repair the module only after power, ground, and network checks pass.
Typical fixes for U0121
- Replace the blown fuse and correct the cause
- Clean or repair the ABS connector
- Repair ground or CAN wiring
- Repair or replace the ABS control module, with programming if required
Replace parts only after testing confirms the cause — a code names the circuit or condition that failed a check, not necessarily the faulty part. We don’t publish per-repair prices; for what households actually spend on upkeep, see car repair costs.
Related codes & warning lights
U0121 FAQ
What does code U0121 mean?
U0121 means other control modules stopped receiving network messages from the anti-lock brake system (ABS) control module. The standard definition is "Lost Communication With Anti-Lock Brake System (ABS) Control Module."
Can I drive with a U0121 code?
Normal braking usually still works, but ABS, traction control, and stability control may be disabled. Drive gently with extra stopping distance, especially on wet or slippery roads, and get it repaired promptly; if the red brake warning light is on or the pedal feels wrong, do not drive.
Will my brakes still work with U0121?
Base hydraulic brakes typically still work, but anti-lock function and stability aids may be off, so wheels can lock in a hard stop.
Why did my speedometer stop working with U0121?
On many vehicles, the ABS module supplies vehicle speed data to the network, so losing it can affect the speedometer and cruise control.
Keep researching
More network communication (u-codes) codes
Sources
- SAE J2012 — Diagnostic Trouble Code Definitions (standard code titles)
- ISO 15031-6 — Road vehicles: communication between vehicle and external equipment for emissions-related diagnostics, Part 6: Diagnostic trouble code definitions
- U.S. EPA — Vehicle Emissions Inspection and Maintenance (I/M) programs
- U.S. EPA, 40 CFR 86.1806-17 — Federal onboard diagnostics requirements for light-duty vehicles
General information written by CarWhere Research, not a diagnosis of your vehicle. Manufacturers can add their own sub-definitions and test conditions; confirm with the factory service information for your year, make and model. Before paying for a repair, check NHTSA for open recalls and your warranty booklet: federal law requires an emissions warranty on major emission-control parts such as the catalytic converter and engine computer.