U0121Network Communication

Lost Communication With ABS Control Module

U0121 means a module has lost communication with the anti-lock brake system control module over the CAN bus. Safety systems are disabled. It usually points to a wiring, power, ground, or connector fault.

🚗 Not recommended. Losing the ABS module disables anti-lock braking, stability control, and traction control.

HIGH

severity

🎯 Top Causes, Ranked by Likelihood

#1Wiring, connector, or CAN bus fault$50-$500

45%

Damaged or corroded wiring to the ABS module.

#2Power or ground fault$50-$400

30%

Blown fuse or poor ground at the ABS module.

#3Failed ABS control module$400-$1500

15%

The ABS module itself has failed.

🩺 Common Symptoms

ABS warning light
Stability and traction control disabled
Speedometer problems
Warning lights
Other U-codes stored

💰 Repair Cost: DIY vs Shop

DIY Cost

$50-$400

Shop Cost

$150-$1200

📋 Free OBD2 Code Cheat Sheet

The 50 most common codes with likely cause and DIY fix cost. Sent once, no spam.

🔧 How to Diagnose U0121

1Check fuses, power, and grounds for the ABS module

2Inspect CAN wiring and connectors at the module

3Scan all modules for lost-communication patterns

4Back-probe the CAN bus for correct signal

❓ Frequently Asked Questions

Can I drive with U0121?

Not safely. ABS and stability control are disabled, which is dangerous in emergency braking. Diagnose it first.

What causes U0121?

Wiring, connector, or power/ground faults at the ABS module, far more often than a failed module.

Do my regular brakes still work with U0121?

Base hydraulic braking usually still works, but anti-lock, stability, and traction control are off. Drive with care and repair soon.

🔗 Related Codes

U0100U0122U0128U0155