Fault codes › Network › Generic (SAE standard) › communication between control modules
U0121 Lost communication with ABS control module
Urgent Risk of damage or of being stranded. Stop, or get it seen, now.
What does U0121 mean?
U0121 is a generic network code, defined by the SAE standard, so it means the same thing on every make and model: lost communication with ABS control module. It belongs to the communication between control modules group of codes.
A control module stores the code when something it watches stays outside what it expects for long enough. On its own the code says which system complained, not which part has failed — the causes below, most likely first, are where to start.
Is it safe to drive with U0121?
Urgent. Braking still works but ABS and stability control may not — drive accordingly and fix promptly.
Common causes of U0121
- CAN wiring fault.
- ABS module failure.
- Blown fuse.
Symptoms
- ABS, traction and stability warnings.
- No ABS assistance.
What to check first
- Fuses.
- Bus wiring to the ABS module.
- Module power.
Can I just clear U0121?
You can, and the light goes out — but a fault that is still there sets the code again, often within a drive or two. Clearing also erases the freeze frame (what the engine was doing when the code was stored) and resets the emissions readiness monitors, so an MOT, smog or inspection test will wait until they have run again. Fix the cause first, then clear it, then watch whether it returns.
Read and clear U0121 from your iPhone with OBDTrip
Plug a Bluetooth OBD-II adapter into the car and OBDTrip reads every stored, pending and permanent code, explains each one in plain English like this, shows what the engine was doing when it was set, and clears the check engine light once the fault is fixed. It keeps a history of when each code comes back, and records every drive on its own from then on.
Needs a Bluetooth Low Energy adapter such as the Vgate iCar Pro — which adapters work with iPhone. Reading and clearing codes is part of an OBDTrip subscription.
Related codes
- U0100 Lost communication with engine control module
- U0101 Lost communication with transmission control module
General guidance from OBDTrip's fault-code catalogue, not a diagnosis of your car. When a code points to brakes, steering, overheating or a flashing check engine light, have the car looked at before driving on.