U0121: Lost communication with the ABS control module

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.

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 this same explanation and whether it’s confirmed, pending or permanent.

With Free you read this code with its short description for nothing, and clear it if the engine stores it over standard OBD. With Pro you get the full explanation, how the car was running when it was logged (revs, temperature, load) and clearing in every control unit.

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 the ITV. It’s all in how to read and clear fault codes and in the ITV and OBD.

Similar codes

• U0102 Lost communication with transfer case control module
• U0103 Lost communication with the gear shift module
• U0104 Lost communication with the cruise control module
• U0107 Lost communication with the throttle actuator control module
• U0126 Lost communication with the steering angle sensor module
• U0128 Lost communication with the park brake control module
• U0131 Lost communication with the power steering control module
• U0140 Lost communication with the body control module (BCM)
• U0100 Lost communication with engine control module (ECM/PCM A)
• U0101 Lost communication with transmission control module (TCM)
• U0141 Lost communication with body control module A
• U0146 Lost communication with gateway A

U0121 is a generic code, so it means the same on any make, for example on Audi, Fiat, Kia, Mazda, Opel / Vauxhall, Toyota. See what Final OBD does on your car in makes and models.