Front Fog Lamps Malfunction

Front Fog Lamps Malfunction

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. Initial Inspection and Diagnostic Overview
* Review Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions provides an overview of each diagnostic category.

Circuit/System Description

The front fog lamp switch signal circuit is referenced from battery voltage by the BCM. When the fog lamp switch is activated, the signal circuit is grounded through the switch and the signal circuit voltage goes low. The BCM controls the front fog lamps through the fog lamp relay in the underhood fuse block. Battery voltage is supplied to the relay coil and switch, and the BCM energizes the relay by grounding the fog lamp relay control circuit.

Circuit/System Verification

1. Ignition ON, observe the scan tool Front Fog Lamp Switch parameter while pressing and releasing the front fog lamp switch. The reading should change between Active and Inactive.

^ If not the specified value, refer to the fog lamp switch circuit test.

2. Ignition ON, command the Fog Lamps ON and OFF with a scan tool. Verify the front fog lamps turn on and off.

^ If the front fog lamps are always on or always off, refer to the fog lamp control circuit test.

Circuit/System Testing

Fog Lamp Switch Circuit Test
1. Ignition OFF, disconnect the harness connector at the headlamp switch.
2. Test for less than 1 ohm of resistance between the ground circuit terminal 8 and ground.

^ If greater than the specified range, test the ground circuit for an open/high resistance.

3. Ignition ON, verify the scan tool Front Fog Lamp Switch parameter is Inactive.

^ If not the specified value, test the signal circuit terminal 10 for a short to ground. If the circuit tests normal, replace the BCM.

4. Install a 3A fused jumper wire between the signal circuit terminal 10 and the ground circuit terminal 8. Verify the scan tool Front Fog Lamp Switch parameter is Active.

^ If not the specified value, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

5. If all circuits test normal, test or replace the headlamp switch.

Fog Lamp Control Circuit Test
1. Ignition OFF, disconnect the fog lamp relay.
2. Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal 87 and ground.

^ If the test lamp illuminates, test the fog lamp control circuit for a short to voltage.

3. Verify that a test lamp illuminates between the B+ circuit terminal 30 and ground.

^ If the test lamp does not illuminate, replace the underhood fuse block.

4. Verify that a test lamp illuminates between the B+ circuit terminal 85 and ground.

^ If the test lamp does not illuminate, replace the underhood fuse block.

5. Disconnect the harness connector at the affected fog lamp.
6. Test for less than 1 ohm of resistance between the fog lamp ground circuit terminal B and ground.

^ If greater than the specified range, test the ground circuit for an open/high resistance.

7. Connect the harness connector at the fog lamp.
8. Connect a 15A fused jumper wire between the B+ circuit terminal 30 and the control circuit terminal 87. Verify the fog lamps are activated.

^ If the fog lamps do not activate, test the control circuit for an open/high resistance. If the circuit tests normal, test or replace the affected fog lamp.

9. Connect a test lamp between the B+ circuit terminal 85 and the relay control circuit terminal 86.
10. Command the Fog Lamps ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.

^ If the test lamp is always ON, test the relay control circuit for a short to ground. If the circuit tests normal, replace the BCM.

^ If the test lamp is always OFF, test the control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

11. If all circuits test normal, test or replace the fog lamp relay.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure. Verification Tests

* Front Fog Lamp Bulb Replacement (Yukon) Front Fog Lamp Bulb Replacement (Escalade)
* Fog Lamp Switch Replacement