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Home >> Cadillac >> 2021 >> CT4 Sport, 4WD >> Repair and Diagnosis >> External Pages >> Different car >> Section 338 (Lighting System - Diagnostic Information And Procedures (2 Of 2)) >> Diagnostic Information and Procedures >> Rear Fog Lamps Malfunction >> Circuit/System Testing

Circuit/System Testing

WARNING: This page is about a different car, the 2022 Cadillac CT5. However, it is still accessible from the selected car via links, so may be relevant.

Rear Fog Lamp Switch Malfunction

  1. Ignition/vehicle off, disconnect the connector at the S78 Turn Signal 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, test for 11.5 - 12.5 V between signal circuit terminal 8 and ground.
    • If less than 11.5 V 
    1. Test circuit 187 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. Test circuit 187 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.
    3. Replace the K9 Body Control Module.
    • If greater than 12.5 V 
    1. Test circuit 187 for a short to voltage.
      • If a short to voltage is found, repair the fault on the circuit.
      • Go to next step: If a short to voltage is not found.
    2. Replace the K9 Body Control Module.
    • Go to next step: If 11.5 - 12.5 V 
  4. Replace the S78 Turn Signal Switch.

Rear Fog Lamps Control Malfunction

  1. Ignition/vehicle OFF, disconnect the E83 Rear Fascia Auxiliary Signal Lamp - Backup/Fog.
  2. Verify a test lamp illuminates between ground circuit terminal 1 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. Connect a test lamp between control circuit terminal 3 and ground, ignition on/vehicle in service mode.
  4. Verify the test lamp turns on and off when commanding the Rear Fog Lamp(s) on and off with a scan tool.
    • If the test lamp is always OFF 
    1. Test circuit 122 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. Test circuit 122 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
    3. Replace the K219 Lighting Control Module.
    • If the test lamp is always ON 
    1. Test circuit 122 for a short to voltage.
      • If a short to voltage is found, repair the fault on the circuit.
      • Go to next step: If a short to voltage is not found.
    2. Replace the K219 Lighting Control Module.
    • Go to next step: If the test lamp turns ON and OFF 
  5. Replace the E83 Rear Fascia Auxiliary Signal Lamp - Backup/Fog.
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