U0121 — Lost Communication with ABS Control Module
The CAN messages from the ABS module are missing. ABS, ESP/ESC and traction control all become inactive (warning lamps illuminate), the speedometer may go to zero on cars that derive vehicle speed from the ABS module, and adaptive cruise control is disabled.
General code reference. Confirm the definition and diagnostic procedure for your vehicle's year, model and engine. A stored code alone does not identify a failed part.
Affected components
- Abs control module
- Can bus wiring
Possible causes to check
These are possible checks, not a ranked diagnosis. Follow the vehicle manufacturer's test procedure before replacing parts.
- ABS module unpowered (often a corroded ground or fuse)
- CAN wiring to ABS broken
- ABS module failed
Possible symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
Diagnostic notes
- The malfunction indicator lamp may illuminate
Sources
Check each reference's vehicle and model-year scope. General background links do not establish which fault is most likely on your vehicle.
Diagnostic information is provided for reference only. Confirm the code with a compatible scan tool and follow the vehicle manufacturer's service procedures before replacing components.