For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provides an overview of each diagnostic category.

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
Rear Fog Lamp B+ | 1 | 1 | 1 | --
Rear Fog Lamp Switch Signal | 2 | 2 | 2 | --
Rear Fog Lamp Relay Control | 1 | 1 | 1 | --
Rear Fog Lamp Control | 1 | 1 | 1 | --
Rear Fog Lamp Ground | -- | 1 | 1 | --
Rear Fog Lamp Switch Ground | -- | 2 | 2 | --
1. Rear Fog Lamp Malfunction 2. Rear Fog Lamp Switch Malfunction

### Circuit/System Description

When the rear fog lamp switch is placed in the ON position, ground is applied through the rear fog lamp switch signal circuit at G300 to the body control module (BCM). The BCM applies ground through the rear fog lamp relay control circuit to the coil side of the fog lamp relay. Battery voltage is applied at all times from the REAR FOG LAMP fuse to the coil and switch sides of the fog lamp relay. When the relay coil is energized, the relay switch contacts close and allow battery voltage to be applied from the REAR FOG LAMP fuse, through the relay switch contacts, to the rear fog lamp. Ground for the rear fog lamp is applied at all times at G402.

### Reference Information

#### Schematic Reference

Fog Lights Schematics

#### Connector End View Reference

Component Connector End Views

#### Description and Operation

Exterior Lighting Systems Description and Operation

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Scan Tool Reference

Control Module References for scan tool information

### Circuit/System Verification

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

⇒ | If the parameter does not change between the commanded states, refer to Rear Fog Lamp Switch Malfunction.

⇒ | If the parameter changes between the commanded states, refer to Rear Fog Lamps Malfunction.

### Circuit/System Testing

#### Rear Fog Lamp Switch Malfunction

1. Ignition OFF, disconnect the harness connector at the rear fog lamp switch.
2. Ignition OFF, verify that a test lamp illuminates between B+ and the ground circuit terminal A.

⇒ | If the test lamp does not illuminate, test the ground circuit for an open/high resistance.

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

⇒ | If not the specified value, test the signal circuit terminal C 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 C and ground. Verify the scan tool Rear 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 rear fog lamp switch.

#### Rear Fog Lamp Malfunction

1. Ignition OFF, disconnect the rear fog lamp relay.
2. Ignition OFF, verify that a test lamp does not illuminate between the control circuit terminal 30 and ground.

⇒ | If the test lamp illuminates, test the control circuit for a short to voltage.

3. Verify that a test lamp illuminates between the B+ circuit terminal 87 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 control circuit terminal 30 for a short to ground. If the circuit tests normal, test or replace the rear fog lamp relay.

4. Verify that a test lamp illuminates between the B+ circuit terminal 86 and ground.

⇒ | If the test lamp does not illuminate, test the B+ circuit for an open/high resistance.

5. Disconnect the harness connector at the rear fog lamp.
6. Ignition OFF, exterior lamps OFF, test for less than 5 ohms between the ground circuit terminal 2 and ground.

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

7. Connect the harness connector at the rear fog lamp.
8. Connect a 10A fused jumper wire between the B+ circuit terminal 87 and the control circuit terminal 30. Verify the Rear Fog Lamp illuminates.

⇒ | If the rear fog lamp does not illuminate, test the control circuit for an open/high resistance. If the circuit tests normal, test or replace the rear fog lamp.

9. Connect a test lamp between the B+ circuit terminal 86 and the control circuit terminal 85.
10. Command the Rear 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 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 rear fog lamp relay.

#### Rear Fog Lamp Indicator Malfunction

Ignition ON, with a scan tool perform the Display Test for the instrument panel cluster (IPC) and observe the rear fog lamp indicator.

⇒ | If the rear fog lamp indicator illuminates during the test, replace the BCM.

⇒ | If the rear fog lamp indicator does not illuminate during the test or remains illuminated at all times, replace the IPC.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Rear Fog Lamp Switch Replacement

• | Relay Replacement

• | Control Module References for BCM replacement, setup, and programming