U0121: Lost Communication With Anti-Lock Brake System (ABS) Control Module
Modules lost contact with the ABS module.
Green, yellow, red: three ways U0121 can look
The same code can be a quick fix or a reason to pull over. Match your situation to the closest example.
Keep an eye on it
Logged once.
Schedule a repair
ABS light on. Regular brakes still work, but get it fixed soon.
Stop and get help
Brake pedal feels wrong or brakes behave oddly. Don't drive.
Symptoms you might notice
- ABS and traction lights
- Stability control off
Common causes, cheapest first
- ABS module power or ground
- Network wiring
- Failed ABS module
What you can check before calling
- Check ABS fuses
A code points to a system, not a part. Replacing parts based on the code alone is how people spend money without fixing the problem. Testing confirms the cause first.
Generic codes (second character 0) mean the same thing on every vehicle. Some manufacturers add their own details, so check service information for your exact vehicle.
Related codes
More network & modules codes
Look up another code
Cummins fault code (like 1922), J1939 SPN and FMI (like SPN 3251 FMI 0), or an OBD-II code (like P0420).
Questions
What does U0121 mean?
U0121 means "Lost Communication With Anti-Lock Brake System (ABS) Control Module". Modules lost contact with the ABS module.
Can I drive with code U0121?
It depends on what the vehicle is doing. Green: Logged once. Yellow: ABS light on. Regular brakes still work, but get it fixed soon. Red: Brake pedal feels wrong or brakes behave oddly. Don't drive.
What causes U0121?
The most common causes are: aBS module power or ground; network wiring; failed ABS module.
Is U0121 a serious code?
It can be. Brake pedal feels wrong or brakes behave oddly. Don't drive.
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