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 has overviews of all diagnostic categories.

DTC Descriptors

DTC B3891 02

Trailer Fog Lamps Circuit Short to Ground

DTC B3891 04

Trailer Fog Lamps Circuit Open

Diagnostic Fault Information

Circuit	Short to Ground	Open/High Resistance	Short to Voltage	Signal Performance
Trailer Fog Lamps Control	3891 02	3891 04	-	-

Circuit/System Description

The front fog lamp switch signal circuit is grounded momentarily by pressing the front fog lamp switch. The body control module (BCM) sends this message to the trailer lighting converter. The trailer lighting converter then applies a voltage to the trailer fog lamps through there supply circuit.

Conditions for Running the DTC

B3891 02 or B3891 04

The appropriate trailer fog lamps signal is active.

Conditions for Setting the DTC

B3891 02

The trailer lighting converter detects a short to ground on the control circuit.

B3891 04

The trailer lighting converter detects an open control circuit.

Conditions for Clearing the DTC

The condition for setting the DTC is not longer present.

Action Taken When the DTC Sets

The appropriate trailer fog lamp is inoperative.

Reference Information

Schematic Reference

Trailer Connector/Provision 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 information on diagnostic tools

Circuit/System Verification

Ignition ON, perform the Trailer Fog Lamp(s) test. The trailer fog lamps should be ON/OFF.

Circuit/System Testing

1.	Ignition OFF, test for less than 5 Ω between the X88 Trailer Connector terminal 3, 11, 13 and ground.

2.	If greater than the specified value, test the appropriate ground circuit for an open circuit/high resistance.

3.	Connect a test lamp between the X88 Trailer Connector terminal 2 and ground.

4.	Command the Trailer Fog Lamp(s) Active/Inactive test with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.

5.	If the test lamp is always ON, test the signal circuit for a short to voltage. If the circuit tests normal, replace the K68 Trailer Lighting Control Module.

6.	If the test lamp is always OFF, test the signal circuit for a short to ground or an open/high resistance. If the circuits test normal, replace the K68 Trailer Lighting Control Module.

7.	If all the circuits test normal, test the fog lamps circuit at the trailer.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for trailer lighting converter replacement, setup and programming