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.
High severity. Can I keep driving?Treat this as a safety fault: drive slowly, leave more distance, and avoid rain or highways until it is repaired, because ABS and stability control may not work. If the pedal feels different or there is a red brake warning, stop and call a tow truck.
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
Most likely causes
- ABS module unpowered (often a corroded ground or fuse)
- CAN wiring to ABS broken
- ABS module failed
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: hard
What each character of U0121 means
- U: Communication network
- 0: Generic code: same on every make
- 1: Subsystem depends on the code range
- 21: Specific fault in that system
Frequently asked questions
Can I keep driving with code U0121?
Treat this as a safety fault: drive slowly, leave more distance, and avoid rain or highways until it is repaired, because ABS and stability control may not work. If the pedal feels different or there is a red brake warning, stop and call a tow truck.
What causes code U0121?
The most common causes are: aBS module unpowered (often a corroded ground or fuse); cAN wiring to ABS broken; aBS module failed.
Sources
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.