Is It Safe to Drive With a U0121 Code?
Common Symptoms of a U0121 Fault
ABS warning light
Traction control warning light
EML on
ABS and stability systems disabled
Multiple warning lights
Possible Causes of U0121
A U0121 code can be triggered by faults across several systems. Identifying the exact root cause requires proper diagnostic equipment and live data — not just reading the code.
CAN Bus Wiring to ABS Module
CAN bus wiring to the ABS module is damaged.
ABS Module Power / Ground Fault
Poor power or earth to the ABS module.
ABS Module Failure
The ABS control module itself has failed.
Battery / Voltage Issue
Low voltage causing ABS module communication loss.
Why we don't list the exact part: A U0121 code tells you what's happening but not why. Replacing parts based on the code alone is the most common way drivers waste money. A proper diagnosis identifies the specific cause first.
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U0121 and Your MOT
A stored U0121 fault code will cause your car to fail its MOT. The engine management light will be illuminated during the test — an automatic failure under current MOT standards. Even if the light temporarily clears, the code remains in ECU memory and can be read by testers.
Get the fault diagnosed and repaired first, then have the code cleared and confirm the light stays off before booking your MOT.