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Pinpoint Test A : U0121

  1. Normal Operation and Fault Conditions 

    REFER to: Communications Network - System Operation and Component Description .

    DTC Fault Trigger Conditions 

    DTC Description Fault Trigger Condition
    PCM U0121:00 Lost Communication With Anti-Lock Brake System (ABS) Control Module "A": No Sub Type Information The PCM sets this DTC if data messages from the ABS module through the GWM are missing.

    Possible Sources 

    • Communications network concern
    • ABS module
    • GWM
    • PCM

  2. A1 VERIFY THE CUSTOMER CONCERN

    • Ignition ON.

    • Verify there is an observable symptom present.

    Is an observable symptom present? 

    Yes  GO to  A2
    No  The system is operating normally at this time. The DTC may have been set due to high network traffic or an intermittent fault condition.

  3. A2 CHECK THE COMMUNICATION NETWORK

    • Using a diagnostic scan tool, perform a network test.

    Did the ABS module pass the network test? 

    Yes  GO to  A3
    No  REFER to: Communications Network .

  4. A3 PERFORM ABS (ANTI-LOCK BRAKE SYSTEM) CONTROL MODULE SELF-TEST

    • Using a diagnostic scan tool, perform a ABS control module self-test.

    Are any Diagnostic Trouble Codes (DTCs) recorded? 

    Yes  REFER to: Anti-Lock Brake System (ABS) and Stability Control .
    No  GO to  A4

  5. A4 CHECK THE GWM (GATEWAY MODULE A) DIAGNOSTIC TROUBLE CODES (DTCS)

    • Using a diagnostic scan tool, retrieve the GWM Diagnostic Trouble Codes (DTCs).

    Are any Diagnostic Trouble Codes (DTCs) recorded? 

    Yes  REFER to: Communications Network .
    No  GO to  A5

  6. A5 PERFORM THE PCM (POWERTRAIN CONTROL MODULE) SELF-TEST

    • Using a diagnostic scan tool, perform the PCM self-test.

    Are any non-network Diagnostic Trouble Codes (DTCs) present? 

    Yes  REFER to Diagnostic Trouble Code (DTC) Chart  .
    No  GO to  A6

  7. A6 RECHECK THE PCM (POWERTRAIN CONTROL MODULE) DIAGNOSTIC TROUBLE CODES (DTCS)

    NOTE: If new modules were installed prior to the DTC being set, the module configuration may be incorrectly set during the PMI, or the PMI may not have been carried out.

    • Using a diagnostic scan tool, clear the Diagnostic Trouble Codes (DTCs).

    • Wait 10 seconds.

    • Repeat the PCM self-test.

    Is DTC U0121 still present? 

    Yes  GO to  A7
    No  The system is operating correctly at this time. The DTC may have been set due to high network traffic or an intermittent fault condition.

  8. A7 CHECK FOR OTHER CAUSES OF COMMUNICATION NETWORK CONCERN

    NOTE: If new modules were installed prior to the DTC being set, the module configuration can be incorrectly set during the PMI or the PMI may not have been carried out.

    • CHECK the vehicle service history for recent service actions related to the ABS, GWM or PCM. If recent service history is found:

      • verify correct replacement module was installed

        • HVBOM may be used to verify correct part fitment

      • verify the configuration of replacement module was correct

        • re-configure module using as-built data if prior configuration is suspect

      • verify the module was not obtained from a like vehicle and installed into customer vehicle

        • return the swapped module to source vehicle and obtain new replacement module

    • Operate the system and determine if the observable symptom is still present.

    Is the observable symptom still present? 

    Yes  GO to  A8
    No  The system is operating correctly at this time. The concern may have been due to incorrect parts replacement procedures or incorrect module configuration.

  9. A8 CHECK FOR CORRECT ABS (ANTI-LOCK BRAKE SYSTEM) MODULE OPERATION

    • Ignition OFF.

    • Disconnect and inspect the ABS control module connector.

    • Repair:

      • corrosion (install new connector or terminals - clean module pins)
      • damaged or bent pins - install new terminals/pins
      • pushed-out pins - install new pins as necessary

    • Reconnect the ABS control module connector. Make sure they seat and latch correctly.

    • Operate the system and determine if the concern is still present.

    Is the concern still present? 

    Yes  CHECK OASIS for any applicable service articles: TSB, GSB, SSM or FSA. If a service article exists for this concern, DISCONTINUE this test and FOLLOW the service article instructions. If no service articles address this concern, INSTALL a new ABS module. REFER to: the appropriate Service Information. .
    No  The system is operating correctly at this time. The concern may have been caused by module connections. ADDRESS the root cause of any connector or pin issues.
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