Trouble code · Network · Generic (all makes)

U0121 Lost Communication With ABS Control Module

Another computer in the car can't talk to the anti-lock brake (ABS) module over the car's network.

Safe to drive?
Drive with caution
Get it fixed soon
Scanner you need
Multi-system scanner
A basic reader isn’t enough
NHTSA complaints quoting it
27
Vehicle complaints, 2015–2025

Common causes

  • Weak battery or poor ground connections
  • Damaged network wiring or connectors
  • Blown ABS fuse
  • Failed ABS module (sometimes from corrosion)

Symptoms

  • ABS, traction control and stability control lights on
  • Speedometer problems on some cars
  • Cruise control not working

Can I drive with it?

Normal braking usually still works, but ABS and stability control won't. Drive carefully and get it checked soon.

Tip: On many cars the ABS module sits low near the front, where water and road salt get in – check its connector for corrosion.

Which scanner reads it?

Reading U codes and the ABS module's own codes needs a multi-system scanner that covers ABS on your make.

See ABS, Airbag and Transmission Codes for which tools read each system. Not sure which fits you? Try the Find My Scanner quiz.

In the data

Of the vehicle complaints owners filed with NHTSA from 2015 to 2025, 27 quote U0121 – most often from Chevrolet, Ford, Volkswagen owners. Complaints lean toward serious problems, so treat this as a sign of how often the code turns up in real reports, not how often cars get it.

Both sources are compared on the Trends page.

General information to help you understand a code – not a diagnosis. Your car’s service information or a qualified mechanic has the final word. See how we research.