Perform the Diagnostic System Check - Lighting Systems prior to using this diagnostic procedure.

When the ignition switch is in the ON position and the headlamp switch is in either PARK or HEAD position, the fog lights illuminate whenever you press the fog lamp switch. The FOG RLY fuse in the top of the engine fuse block supplies battery positive voltage to both the fog lamp relay switch contacts and to the fog lamp relay coil circuit. The fog lamp relay coil is controlled by the body control module (BCM). Whenever you press the fog lamp switch, the BCM fog lamp switch input is grounded. The BCM energizes the control circuit of the fog lamp relay coil. The current flows from the fog lamp relay to both front fog lamps and to ground G101. The fog lamps remain ON until you again press the front fog lamp switch, or until you cycle the ignition switch OFF and ON. The park lamps or low beam headlamps must be ON in order for the fog lamps to be illuminated. Fog lamp operation will be cancelled whenever the park lamps are turned OFF or the high beam headlamps have been selected.

Schematic Reference

Exterior Lights Schematics

Connector End View Reference

Lighting Systems Connector End Views

Electrical Information Reference

Scan Tool Reference

Ignition ON, observe the Fog Lamp Switch Input parameter while toggling the fog lamp switch ON and OFF. The reading should change between active and inactive.

Fog Lamp switch Input parameter does not toggle between active and inactive

1. Ignition ON, observe the Fog Lamp Switch Input parameter while turning the fog lamp switch ON and OFF. The reading should change between active and inactive.
2. Test for less than 1.0 ohm of resistance between the ground circuit terminal A and ground.

⇒	If greater than the specified range, test the ground circuit for an open/high resistance or short to voltage.

3. Connect a volt meter between the control circuit terminal D and the ground circuit terminal A. The volt meter should display 5 volts.

⇒	If the voltmeter always displays 0 volts, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the body control module.

4. If all circuits test normal, replace the fog lamp switch.

Fog Lamp switch Input parameter does toggle between active and inactive

1. Ignition OFF, disconnect the fog lamps relay.
2. Verify that a test lamp illuminates between the coil side B+ circuit and ground.

⇒	If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal and the B+ circuit fuse is open, replace the fog lamp relay.

3. Verify that a test lamp illuminates between the switch side B+ circuit and ground.

⇒	If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal and the B+ circuit fuse is open, test the fog lamp supply voltage circuit for a short to ground.

4. Disconnect the harness connector at the appropriate fog lamp.
5. Test for less than 1.0 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.

6. Connect the harness connector at the fog lamp.
7. Connect a 10A fused jumper wire between the B+ circuit and the fog lamp supply voltage circuit. Verify the fog lamps illuminate.

⇒	If the fog lamps do not illuminate, test the control circuit for an open/high resistance. If the circuit tests normal, replace the fog lamp bulbs.

8. Connect a test lamp between the B+ circuit and the fog lamp relay control circuit. 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 control circuit for a short to ground. If the circuit tests normal, replace the body control module.

⇒	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 body control module.

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