U0121 on GMC
Lost Communication with ABS Module
U0121 on GMC vehicles indicates lost communication with abs module. Au7o has documented this code across 1 GMC model — most commonly on Sierra 3500HD. U0121 is a network communication code meaning a control module on the vehicle's data bus has lost communication with the ABS (anti-lock brake system) module. Modern cars share information over a CAN bus, and when other modules stop receiving expected messages from the ABS unit, this code sets. Because the ABS module also handles traction control and stability control on most vehicles, you may see multiple warning lights and lost features. It points to a communication or power problem with the ABS module rather than a worn brake part. Typical repair costs on GMC range from $80 to $900, depending on the specific model and root cause.
Common Causes of U0121
- •Loss of power or ground to the ABS module
- •Blown fuse feeding the ABS controller
- •Damaged, corroded, or loose ABS module connector
- •Open, shorted, or chafed CAN bus wiring
- •Internal failure of the ABS control module
- •Corrosion or water intrusion in connectors/harness
- •Faulty gateway module disrupting bus communication
U0121 on GMC by Model
GMC Sierra 3500HD(3 issues)
- Front Differential Actuator Failure Causing 4WD Engagement Problems2007-2022
The front axle disconnect actuator (also called the front axle engagement actuator or IWE—Integrated Wheel End) on four-wheel-drive Sierra 3500HD models is known to fail, preventing proper engagement or disengagement of 4WD. Owners report grinding noises from the front axle area in 2WD mode due to hubs failing to fully disengage, or the inability to engage 4WD when needed. Vacuum leaks in the actuator system are the most common root cause.
- Trailer brake controller / integrated trailer brake module malfunction2015-2023
Sierra 3500HD owners towing heavy loads report intermittent trailer brake output loss, warning messages, or erratic gain behavior from the integrated trailer brake controller. NHTSA complaints and GM service bulletins/diagnostics commonly point to module/software issues, poor connector pin fit/corrosion at the 7-way, or wiring faults that cause the system to disable trailer brakes unexpectedly.
- Electric power steering (EPS) assist loss / reduced assist warnings (new body style)2020-2024
On 2020+ Sierra 3500HD (T1 platform) with electric power steering, owners report intermittent loss of steering assist and warning messages such as "Steering Assist Reduced". NHTSA complaints and GM service information commonly point to EPS module/software issues, wiring/connector concerns, or rack-related faults that can occur without prior symptoms.
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View U0121 across all makes →Frequently Asked Questions
What does U0121 mean on GMC?▼
U0121 stands for "Lost Communication with ABS Module." U0121 is a network communication code meaning a control module on the vehicle's data bus has lost communication with the ABS (anti-lock brake system) module. Modern cars share information over a CAN bus, and when other modules stop receiving expected messages from the ABS unit, this code sets. Because the ABS module also handles traction control and stability control on most vehicles, you may see multiple warning lights and lost features. It points to a communication or power problem with the ABS module rather than a worn brake part. On GMC specifically, this code is documented across 1 model.
What causes U0121 on GMC vehicles?▼
Common causes on GMC: Loss of power or ground to the ABS module, Blown fuse feeding the ABS controller, Damaged, corroded, or loose ABS module connector, Open, shorted, or chafed CAN bus wiring, Internal failure of the ABS control module. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix U0121 on a GMC?▼
Repair costs on GMC range from $80 to $900, depending on the specific model and root cause.
Which GMC models have U0121 documented?▼
Au7o has documented U0121 on 1 GMC model: Sierra 3500HD.