U0121 – Lost communication with ABS control module

U0121 Lost communication with anti-lock brake system (ABS) control module – a generic network code set when another module stops receiving data from the ABS module.

Typical symptoms

• ABS, ESP and traction control warnings
• Speedometer or cruise control not working
• Other modules storing communication codes

Common causes

• ABS module supply fuse or earth fault
• Corroded or loose ABS connector
• Damaged network (CAN) wiring
• Low battery voltage
• Internal ABS module failure

Checks before suspecting the module

1. Check battery voltage
2. Check ABS fuses and earth
3. Inspect the connector for water and corrosion
4. Check whether the module communicates with a scan tool
5. Measure network wiring if other modules are also affected

If supply, earth and wiring are good and communication is still missing, the ABS module may be faulty. Further diagnosis is needed before deciding to repair or replace it. See ABS & ESP module repair.

Sources

• RepairPal – U0121
• Kelley Blue Book – U0121