U0121: Lost communication with the ABS control module
The vehicle network is not receiving messages from the ABS control unit. ABS and stability control switch off, though normal braking still works.
| Code | U0121 |
|---|---|
| System | Communication network |
| Group | U01xx, Lost communication with control units |
| Type | Generic: it means the same on every make |
| Can you keep driving? | With care |
What U0121 means
ABS stops the wheels from locking under hard braking, and its control unit usually also handles stability and traction control. This code means that unit has stopped talking to the others over the data network. While the fault is present, those aids are switched off.
Most common causes
- Low battery or loose terminals
- Blown main ABS fuse (there is often one for the pump and one for the control unit)
- Loose, corroded or wet connector on the ABS unit, which sits in the engine bay exposed to moisture
- Poor ground on the ABS control unit
- Damaged network wiring between the ABS and other modules
- Faulty ABS control unit
Symptoms
- ABS, stability control and sometimes brake warning lights on
- The speedometer may stop working on some cars
- Cruise control stops working
- Wheels may lock under hard braking
Can you drive with U0121?
With care The brakes still work, but without ABS or stability control. Drive gently, leave extra distance, and if the red brake warning light also comes on, do not drive.
Open the bonnetthe in-depth explanation
How to check it
With the car stationary, see whether the reader can connect to the ABS module: if it never answers, check its fuses and connector first. If the code only appears when starting, suspect the battery.
How to read and clear U0121 with Final OBD
Plug an OBD2 scanner in under the steering wheel, switch the ignition on and open Final OBD. Under Faults you’ll see U0121 with its description and whether it’s confirmed, pending or permanent.
If the engine stores it over standard OBD, with FREE you read this code with its explanation for nothing and clear it. With PRO you also read the faults of every control unit and clear them in all of them.
Clearing the code only turns the light off: if the cause is still there, it comes back. Clear it once it’s fixed, and not right before an inspection. It’s all in how to read and clear fault codes and in inspections and OBD.
Similar codes
- U011C The rocker arm control unit B is not answering on the car network
- U011D The all wheel drive control unit is not answering on the car network
- U011E The throttle actuator B control unit is not answering on the car network
- U0120 The starter motor generator control unit is not answering on the car network
- U0122 Communication with the stability control unit has been lost
- U0123 The yaw rate sensor module is not answering on the car network
- U0124 The lateral acceleration sensor module is not answering on the car network
- U0125 Communication with the multi-axis acceleration sensor module has been lost
- U0100 Lost communication with engine control module (ECM/PCM A)
- U0101 Lost communication with transmission control module (TCM)
- U0102 Lost communication with transfer case control module
- U0103 Lost communication with the gear shift module
To understand it properly
We explain how that system works, with drawings and no waffle, in our mechanics guide: Brakes.
What does code U0121 mean?
ABS stops the wheels from locking under hard braking, and its control unit usually also handles stability and traction control. This code means that unit has stopped talking to the others over the data network. While the fault is present, those aids are switched off.
Can you drive with U0121?
The brakes still work, but without ABS or stability control. Drive gently, leave extra distance, and if the red brake warning light also comes on, do not drive.
How do you clear U0121?
With an OBD2 scanner and an app like Final OBD, which clears it for free if it’s stored by the engine control unit. But clearing it only turns the light off: if the cause isn’t fixed, U0121 comes back.
Read it on your car
Final OBD reads and clears engine faults through standard OBD for free, each one explained in plain English. With PRO it also reads the faults of every control unit, each make’s own codes included, and clears them in all of them.
