Fault code U0121 – a step-by-step guide to diagnosing and fixing it

Error code U0121 is displayed and the check engine light is on - what does this mean?

DTC U0121 is detected by the OBD/EOBD system when there is a communication fault between the ABS control unit (anti-lock braking system) and other control units in the vehicle. This error code indicates a problem in the CAN bus network that is affecting data transmission between the various electronic systems.

What can cause this error code?

• Defective ABS control unit
• Damaged CAN bus cables
• Corroded or loose plug connections
• Defective fuses in the ABS system
• Faults in the vehicle wiring
• Faulty earth connections
• Defective CAN bus terminating resistor
• Software error in the ABS module

What symptoms occur in parallel with fault code U0121?

• ABS warning light: The ABS indicator light is permanently illuminated in the dashboard
• ESP/ESC malfunction: The Electronic Stability Program is not working properly
• Brake assist failure: Brake aids such as brake assist or emergency brake assist are deactivated
• Traction control: Traction control may not be available
• Faulty speed display: Irregular or missing speed display
• Communication error: Other control units also report communication errors

How to fix the problem of trouble code U0121?

• Repairing trouble code U0121 requires a systematic diagnosis of the CAN bus system.
• Check and replace the ABS control unit: Replace the defective ABS module with a new or overhauled control unit and have it programmed
• Repair CAN bus lines: Locate damaged or interrupted CAN high and CAN low lines and replace with new cables
• Clean and secure plug connections: Disassemble all relevant connectors on the ABS system, clean and treat with contact spray
• Replace fuses and relays: Identify defective fuses in the ABS fuse box and replace with new ones
• Check ground points: Clean and tighten corroded or loose ground connections on the ABS system
• Carry out a software update: Have the ABS control unit updated with the latest firmware
• Measure the CAN bus terminating resistor: Check the 120 ohm resistor in the CAN bus network and replace if necessary