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 diangostic approach.

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

### DTC Descriptors

DTC B3891 02: Trailer Fog Lamps Circuit Short to Ground
DTC B3891 05: Trailer Fog Lamps Circuit Short to Battery or Open

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
Rear Fog Lamp Switch Signal Circuit | B3891 02 | B3891 05 | B3891 05 | --
Rear Fog Lamp Ground Circuit | -- | B3891 05 | -- | --
Rear Fog Lamp Switch Signal Circuit | 1 | 2 | -- | --
Rear Fog Lamp Switch Ground Circuit | -- | 2 | -- | --
1. Fog Lamp Always On - Rear 2. Fog Lamp Inoperative - Rear

### Circuit/System Description

When the rear fog lamp switch is placed in the ON position, ground is applied to the auxiliary body control module (XBCM) through the rear fog lamp switch circuit. The XBCM then applies voltage on the trailer fog lamp circuit to the trailer rear fog lamp.

### Conditions for Running the DTC

The ignition is ON. Park lamps must be ON and the rear fog lamps must be commanded ON.

### Conditions for Setting the DTC

The body control module (BCM) detects an open, short to ground, or short to voltage on the trailer rear fog lamp circuit while trying to apply a voltage to the circuit.

### Action Taken When the DTC Sets

The trailer rear fog lamps do not illuminate.

### Conditions for Clearing the DTC

• | The conditions for setting the DTC are no longer present.

• | A history DTC clears after 100 malfunction-free ignition cycles.

• | The BCM receives the clear code command from the scan tool.

### Reference Information

#### Schematic Reference

Fog Lights Schematics

#### Connector End View Reference

Component Connector End Views

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Scan Tool Reference

• | Scan Tool Output Controls

• | Scan Tool Data List

• | Scan Tool Data Definitions

### Circuit/System Verification

Ignition ON, activate with a scan tool the Trailer Fog Lamps output control for the auxiliary body control module (XBCM). The Rear Fog Lamp Relay Cmd. data parameter should display ON and the trailer rear fog lamps should illuminate.

### Circuit/System Testing

1. Inspect the trailer rear fog lamp bulb for an open or shorted filament.

⇒ | If the bulb is inoperable, replace the bulb.

2. Park lamp switch OFF, clear the DTC. Test the trailer rear fog lamp circuit for a short to voltage, short to ground, an open, or high resistance.

⇒ | If all circuits/connections test normal, replace the XBCM.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for XBCM replacement, setup, and programming