U0121 — Lost Communication With Anti-Lock Brake System (ABS) Control Module

What it means

Messages from the ABS hydraulic control unit are missing on the data bus, so other modules lose the wheel speed data it broadcasts. ABS, traction control, stability control and often hill hold and adaptive cruise are deactivated, and their warning lamps come on together. Base hydraulic braking still works, but without any anti-lock or stability intervention.

Likely causes (most common first)

1. Blown fuse or lost power supply to the ABS unit
2. Corroded or wet connector at the ABS hydraulic unit, a very common failure point in the wheel arch area
3. Broken or chafed CAN wiring on the ABS branch
4. Poor ground connection of the ABS module
5. Internal fault of the ABS control unit, often cracked solder joints on the pump driver stage
6. Bus fault caused by another module or a damaged terminating resistor

Can you drive with it?

Drive carefully and slowly: normal brakes work but ABS and stability control are off, so braking distance on a slippery road grows.