U0121: Lost Communication With Anti-Lock Brake System (ABS) Control Module
Another module has stopped receiving messages from the ABS control module.
High risk
Have it checked soon. ABS and stability control may not work.
Short answer · Can I keep driving with U0121?
Normal braking usually remains, but ABS and stability control may be off. Drive carefully and have it checked.
What does U0121 mean?
U0121 is stored when modules stop receiving messages from the ABS control module, which also supplies wheel speed data to other systems.
What the car does when the code is set
Functions that depend on the affected module are switched off. ABS, stability control, cruise control or power steering assistance may be unavailable, with matching warning lights.
How the system works
The ABS control module and power steering module share wheel speed, steering angle and other data over the network. Other modules rely on this data and store a lost communication code when it stops arriving.
Common symptoms
Possible causes
Listed without ranking. The cause on your car depends on make, engine and mileage — see the maker notes below.
- Power or ground fault at the moduleA blown fuse or poor ground switches the module off.
- Connector corrosionModules near the wheels or under the car are exposed to water and salt.
- Damaged network wiringChafed or corroded CAN wires.
- Low battery voltageVoltage dips can cause temporary communication loss.
- Failed moduleLess common than wiring faults.
How it is diagnosed
- Check which warning lights are onABS, stability control or steering warnings show which system is affected.
- Check fuses and battery voltageStart with the simplest causes.
- Inspect the module connectorLook for corrosion and loose pins.Disconnect the battery only as described in the manual; some modules need a relearn afterwards.
- Scan all modulesA workshop checks whether the module responds and tests the network wiring.
Questions about U0121
Why do the speedometer and cruise control fail too?
They often use wheel speed data sent by the ABS module.
Do the brakes still work?
Normal braking usually remains, but ABS and stability control may be off. Drive carefully and have it checked.