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Home >> GMC >> 2024 >> Sierra EV >> Repair and Diagnosis >> Body & Frame >> Towing Systems >> TRAILERING Systems >> Diagnostic Information and Procedures >> DTC C159C >> Circuit/System Testing

Circuit/System Testing

    NOTE:
    • You must perform the Circuit/System Verification before proceeding with Circuit/System Testing.
    • Refer to Diagnostic Aids before proceeding with Circuit/System Testing.
  1. Ignition/vehicle off, disconnect the connector at the S76 Trailer Brake Control Switch.
  2. Verify a test lamp illuminates between ground circuit terminal 3 and B+.
    • If the test lamp does not illuminate. 
    1. Repair the open/high resistance on the ground circuit or ground connection.
    • Go to next step: If the test lamp illuminates. 
  3. Ignition on/vehicle in service mode.
  4. Verify a test lamp illuminates between B+ circuit terminal 1 and ground.
    • If the test lamp is always off and the circuit fuse is OK. 
    1. Test circuit 3640 for an open/high resistance.
      • If an open/high resistance is found, repair the fault on the circuit.
      • Go to next step: If an open/high resistance is not found.
    2. Verify there is voltage at the fuse.
    • If the test lamp is always off and the circuit fuse is open. 
    1. Test circuit 3640 for a short to ground.
      • If a short to ground is found, repair the fault on the circuit.
      • Go to next step: If a short to ground is not found.
    2. Replace the S76 Trailer Brake Control Switch.
    • Go to next step: If the test lamp illuminates. 
    5. NOTE:

    Refer to Diagnostic Aids before replacing any components.

  5. Replace the S76 Trailer Brake Control Switch.
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