Do the brakes still work with U0121?

The basic hydraulic brakes normally do, but ABS and stability control may be off, so drive with extra care.

Network communication

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

Other modules cannot communicate with the ABS control module. ABS, traction control and stability control may be disabled, but the normal brakes usually still work.

Severity: High

Common causes

CAN bus wiring fault or corroded connector near the ABS module
Faulty ABS control module or its power and ground supply
Blown ABS fuse or relay
Water damage to the ABS module
Low battery voltage

Symptoms

ABS, traction control and stability warning lights on
Speedometer or other gauges affected
Loss of ABS and stability functions
Other U codes

How to check

Check battery voltage and the ABS fuses, relays and grounds
Check CAN wiring and the ABS module connector for corrosion
Scan all modules to see which ones can communicate
Test the ABS module power supply; repair or replace the module if needed

Can you keep driving?

Standard braking usually works, but ABS and stability control are disabled. Drive carefully and repair it soon.

Frequently asked questions

What does U0121 mean?: Other modules cannot communicate with the ABS control module.
Do the brakes still work with U0121?: The basic hydraulic brakes normally do, but ABS and stability control may be off, so drive with extra care.

General reference information. It does not replace a diagnosis on your own vehicle. The same code can have different causes depending on make, model year and engine; check the manufacturer's documentation or ask a technician with a scan tool.