B3891

DTC B3891

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.

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/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.

Circuit/System Verification

Ignition ON, with a scan tool activate the Trailer Fog Lamps. 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. Verification Tests