Fix U0121: Mechanic’s solutions that WORKED

The U0121: Lost Communication With Anti-Lock Brake System (ABS) Control Module signifies a breakdown in communication between the vehicle’s main computer (ECM) and the Anti-Lock Brake System (ABS) control module.

U0121 quick overview

Meaning	U0121: Lost Communication With Anti-Lock Brake System (ABS) Control Module
Is it serious?	This code can be serious as it indicates a communication issue with the vehicle’s safety systems, potentially affecting braking performance.
Possible causes	– Faulty ABS control module – Damaged or corroded wiring and connectors – Issues with the vehicle’s CAN bus communication network – Malfunctioning wheel speed sensors
How to diagnose?	Scan for Codes: Use a diagnostic scanner to confirm the U0121 code and check for other related codes. Inspect Wiring and Connectors: Examine the wiring and connectors related to the ABS module for damage or corrosion. Check ABS Module: Verify the operation and condition of the ABS control module. Evaluate CAN Bus Network: Assess the vehicle’s CAN bus network for issues that could disrupt communication. Wheel Speed Sensor Check: Inspect the wheel speed sensors for damage or malfunction.

How to fix U0121 – learn from mechanics

I looked into U0121 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Chevrolet

Vehicle Information: 2014 Chevrolet Equinox LT 2.4L

Issue + Repair History:

• Customer reported fluctuating gauges and service engine light flashing.
• Found multiple U0121 codes.
• Ground and power connections checked.
• Searched for a TSB-referenced connector without success.

Suggestions to Fix Issue:

• Consider a misfire might be causing the service engine light to flash.
• Check the vehicle’s battery and ground connections, especially in areas with road salt.
• Inspect and clean the vehicle battery post clamps.
• Perform a visual inspection for rust or corrosion at visible ground points.
• Observe data on the scan tool, especially module voltages and communication.
• Disconnect and inspect the harness connector of any non-communicating modules.

Final Fix:

• Replaced the electric steering motor, which resolved the issue.

Case #2: Jeep

Vehicle Information: 2012 Jeep Grand Cherokee Laredo 3.6L

Issue + Repair History:

• Intermittent no start and non-responsive remote control.
• Inserting and wiggling the key sometimes made everything work.
• Ignition switch previously replaced.
• Additional codes found: U0168, U110A.

Suggestions to Fix Issue:

• Check the CAN network and main dash connector for water damage or pin issues.
• Communicate with the Anti-Lock Brake Module using a scan tool; check relays and fuses.
• Inspect the connection/terminal at the ignition switch.
• Consider disconnecting ABS module to test if it’s pulling down the CAN network.
• Verify all wire connections, especially at the new switch.
• Test inputs and outputs at the ignition switch/WIN module.

Final Fix:

• Located a loose male wire connector in the front passenger kick panel.
• Secured the yellow with white stripe wire next to brown with red stripe wire.
• After securing the connection, the issue was resolved.

Case #3: GMC

Vehicle Information: 2012 GMC Terrain SLE 2.4L

Issue + Repair History:

• “Service Stabilitrak” message on instrument panel.
• Codes present: U0074, C0710, U0126, C0287, C0186, C0196.
• Steering angle sensor replaced but did not fix the issue.
• TSB for replacing Yaw Rate Sensor and reprogramming followed.

Suggestions to Fix Issue:

• Check for vehicle history of accidents, which could affect data lines.
• Inspect for spills in the console area, particularly near switches.
• Look for corrosion in connector X411 located under the rear of the vehicle.
• Measure the circumference of all four tires to ensure they are the same.

Final Fix:

• Found software updates for the EBCM.
• Updated controller and drove vehicle in various conditions.
• Stabilitrak light and codes were resolved after the update.

Case #4: Toyota

Vehicle Information: 2006 Toyota RAV4 3.5L

Issue + Repair History:

• Customer reported that the EPS light came on while driving.
• TSB suggested replacing the steering module.
• The steering module was replaced.

Other Test Results:

• Scanned for codes: U0105, U0121.

Suggestions to Fix Issue:

• Replacement module does not require reprogramming.
• Ensure the steering wheel is centered.
• Check the steering angle sensor if possible.

Final Fix:

• The steering module was replaced and was plug-and-play.
• No issues with lights, and the vehicle drives straight without problems.

Case #5: Chevrolet

Vehicle Information: 2012 Chevrolet Silverado 2500 HD WT 6.0L

Issue + Repair History:

• “Service trailer brake system” message displayed on the driver information center.
• Trouble code U0121: lost communication with ABS control module.

Suggestions to Fix Issue:

• Check the trailer module on the frame rail near the rear of the truck for corrosion.
• Inspect the Yaw and Lateral Accelerometer Sensor or Steering Wheel Angle Sensor supply voltage for shorts.
• Check EBCM ground under the driver’s side on the body mount behind the right front wheel.
• Perform a voltage drop test on the power and ground circuits at the EBCM connectors.
• Investigate a recall related to faulty plug-in at BCM under the dash.

Final Fix:

• Replaced the trailer tow relay.

Advice	Description of Diagnostic Step
Check Power and Ground Connections	Ensure proper power and ground to the relevant module (e.g., ABS, EBCM).
Inspect for Corrosion or Damage	Look for corrosion, especially in underbody connectors and modules.
Verify Communication with Affected Modules	Use a scan tool to confirm communication with all related modules.
Perform a Voltage Drop Test	Conduct voltage drop tests on power and ground circuits of the modules.
Look for Relevant Technical Service Bulletins (TSBs)	Search for any TSBs that might apply to the specific vehicle make and model.