U0121 U0xxx · Network (U) High severity
Lost Communication with ABS / ESP Control Module
The ABS module has dropped off the bus. Stability and traction functions are gone, and other modules usually complain too.
Severity guidance: High — stop driving and diagnose now; risk of further damage
Symptoms you will notice
- ABS and ESP lamps on
- ABS module unreachable
- Steering angle and yaw codes alongside
Common causes
Ordered from most to least likely — start at the top.
- 1 ABS module power or earth fault
- 2 Corroded connector at the modulator
- 3 CAN wiring fault
- 4 Failed module
How to diagnose it
Cheapest and least invasive first — most of these codes are not a parts problem.
- ✓ Check the ABS fuses and the high-current supply first.
- ✓ Inspect the modulator connector for corrosion or water.
- ✓ Measure CAN at the ABS connector.
- ✓ Isolate the module to confirm it is not dragging the bus.
Repair options
- Repair the supply or earth
- Repair the CAN wiring
- Replace the module if failed
U0121 — frequently asked questions
Do the brakes still work? +
Base braking remains. You lose ABS and stability intervention — urgent but not immobile.
Why do steering angle codes appear too? +
Other modules depend on data the ABS module normally relays.
Confirm the diagnosis before you buy parts
Send us the code, freeze-frame data and the vehicle — our engineers will tell you what to test first.