Body Control System

U0121-LOST COMMUNICATION WITH ANTI-LOCK BRAKE MODULE

U0121 - Lost Communication With Anti-lock Brake Module:

WHEN MONITORED
- With the ignition on
- Battery voltage between 10 and 16 volts
- IOD fuse installed
- FCM is configured correctly

SET CONDITION
Bus messages not received from the Antilock Brake Module for approximately 2 to 5 seconds.

POSSIBLE CAUSES
- CAN B or CAN C BUS circuits open or shorted
- DTCs related to battery voltage, ignition, or VIN messages
- FCM not configured correctly
- Antilock brake module power and ground
- Antilock brake module
- Module that set this DTC

Diagnostic Test

1. VERIFY DTC IS ACTIVE

NOTE: Ensure the IOD fuse is installed and battery voltage is between 10 and 16 volts before proceeding.

With the scan tool, read active DTCs.

Q: Is this DTC active?

YES: Go To 2
NO: Refer to the Stored Lost Communication test procedure. Stored Lost Communication DTCs

2. CHECK FOR ANY OF THE FOLLOWING ACTIVE DTCS

With the scan tool, read all active DTCs from all modules.

NOTE: Check for FCM configuration, CAN B or C hardware electrical, VIN Missing/Mismatch, battery or ignition related DTCs.

Q: Does the scan tool display any active DTCs to the conditions listed above?

YES: Diagnose and repair the DTC.
NO: Go To 3

3. VERIFY THAT THE ABS IS ACTIVE ON THE BUS

Turn the ignition on.
With the scan tool, select Network Diagnostics.
Verify that the ABS is active on the bus.

Q: Is the ABS active on the bus?

YES: Go To 4
NO: Refer to Manufacturer Code Charts / Communications for the No Response test procedure.

4. CHECK FOR ADDITIONAL COMMUNICATION RELATED DTCS

With the scan tool, select Network Diagnostics.

Q: Is there more than one module with active DTCs "Logged Against" the ABS?

YES: Replace/update the Antilock Brake Module. Perform ABS VERIFICATION TEST - VER 1. ABS Verification Test
NO: Replace/update the module that set this DTC. Perform the appropriate VERIFICATION TEST.