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Pinpoint Test K: U0121:00

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  1. Normal Operation and Fault Conditions 

    REFER to: Exterior Lighting - Overview . REFER to: Exterior Lighting - System Operation and Component Description .

    DTC Fault Trigger Conditions 

    DTC Description Fault Trigger Condition
    HCM U0121:00 Lost Communication With Anti-Lock Brake System (ABS) Control Module "A": No Sub Type Information A continuous memory and on-demand DTC that sets in continuous memory if the HCM does not receive data messages from the ABS module through the GWM are missing for 5 seconds or longer.

    Possible Sources 

    • Communication concern
    • Battery voltage concern
    • ABS module
    • HCM

  2. K1 VERIFY THE CUSTOMER CONCERN

    • Ignition ON.

    • Verify there is an observable symptom present.

    Is an observable symptom present? 

    Yes  GO to  K2
    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. K2 CHECK THE COMMUNICATION NETWORK

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

    Does the ABS module pass the network test? 

    Yes  GO to  K3
    No  REFER to: Exterior Lighting - System Operation and Component Description .

  4. K3 PERFORM THE ABS (ANTI-LOCK BRAKE SYSTEM) MODULE SELF-TEST

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

    Are any Diagnostic Trouble Codes (DTCs) recorded? 

    Yes  REFER to: Anti-Lock Brake System (ABS) and Stability Control .
    No  GO to  K4

  5. K4 RECHECK THE HCM (HEADLAMP 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 HCM self-test.

    Is DTC U0121:00 still present? 

    Yes  GO to  K5
    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.

  6. K5 CHECK FOR DTC (DIAGNOSTIC TROUBLE CODE) U0121:00 SET IN OTHER MODULES

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

    • Ignition OFF.

    • Ignition ON.

    • Wait 10 seconds.

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

    • Retrieve the Continuous Memory Diagnostic Trouble Codes (CMDTCs) from all modules.

    Is DTC U0121:00 set in the BCM? 

    Yes  INSTALL a new ABS module. REFER to: Anti-Lock Brake System (ABS) Module .
    No  INSTALL a new HCM. REFER to: Headlamp Control Module (HCM) .
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