U0121: Lost Communication with ABS Control Module

What Does U0121 Mean?

Code U0121 indicates that a control module has lost CAN bus communication with the Anti-lock Brake System (ABS) control module. The ABS module shares wheel speed data and brake status with other systems including traction control, stability control, and the instrument cluster. Loss of communication disables these integrated safety features.

Common Causes

• Failed ABS control module
• CAN bus wiring fault near the ABS module
• ABS module connector corrosion
• ABS module fuse blown
• Water damage to ABS module or connector

Symptoms

• ABS warning light on
• Traction control light on
• Stability control disabled
• Speedometer may stop working
• Brake warning light on some vehicles

Diagnostic Steps

1. 1Check ABS module fuse and power supply
2. 2Attempt direct communication with the ABS module via scan tool
3. 3Inspect ABS module connector for corrosion or water damage
4. 4Check CAN bus wiring at the ABS module
5. 5Verify ABS module ground connections
6. 6Check for water intrusion in the ABS module area, typically located near the engine bay firewall

Estimated Repair Cost

Frequently Asked Questions