U0121: Lost communication with anti-lock brake system (ABS) module
Another control module, often the engine computer, has lost contact with the ABS module over the network. ABS, traction and stability control may be switched off.
Updated
What U0121 means
The ABS module controls the anti-lock brakes and, on most newer cars, traction and stability control. It shares data with the engine computer and other modules over the network, the CAN bus.
U0121 means a module, often the engine computer, stopped hearing the ABS module. The ABS module may have lost its power, or the network wiring to it has a fault.
Read My Car can read U0121 where the engine computer or the gearbox module stored it, over generic OBD2, but it can’t read the ABS module itself: its own codes need a tool for your make.
Can I keep driving?
Drive gently
Drive gently and have it checked soon. ABS and stability control may be off: the brakes still work, but the wheels can lock in an emergency stop and the car can skid more easily. Leave extra distance, brake early and take extra care on wet or slippery roads.
General guidance based on the code alone, not on the car’s actual condition. If in doubt, stop and ask a mechanic.
With the ABS warning on, the brakes still work as normal brakes, but without anti-lock: in an emergency stop the wheels can lock, and you can’t steer while they do. Brake earlier and more gently, and keep more distance.
If the red brake warning is on as well, or the brake pedal feels different, stop when it’s safe to do so and have the car checked before driving on.
Symptoms
- ABS, traction control and stability control warnings on together.
- On cars that take the road speed from the ABS, the speedometer stops working or reads wrong, and cruise control may not work.
- On some cars, limp mode or odd gear changes.
Common causes
- Loss of power or earth to the ABS module, such as a blown fuse. The ABS module has its own fuses: check them first.
- Network wiring or connector fault. On most cars the ABS unit sits in the engine bay, where its connector is exposed to water and dirt.
- Faulty ABS module. Repaired or replaced only after its power, earth and wiring check out. A replacement usually has to be coded to the car.
Typical fixes
- Check the battery and its connections: low voltage can make modules drop off the network.
- Check the ABS fuses.
- Have the ABS module read with a tool for your make: it often stores the reason itself.
- Have the ABS module’s power, earth and network wiring checked, then the module.
Questions
Can I drive with U0121?
Gently, and have it checked soon. ABS and stability control may be off: the brakes still work, but the wheels can lock in an emergency stop and the car can skid more easily. Leave extra distance, brake early and take extra care on wet or slippery roads.
Do my brakes still work with U0121?
Normally yes, as brakes without ABS, and stability control may be off. If the red brake warning is on too, or the pedal feels different, stop when it’s safe to do so and have the car checked before driving on.
Why can’t Read My Car read the ABS codes?
Generic OBD2 covers the engine and often the gearbox. The ABS module uses the make’s own diagnostics, so its codes need a tool for your make.