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DTC B3891

DTC B3891

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.


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 B3891 02 B3891 04 1 -
1. Trailer Fog Lamps Always ON


Circuit/System Description

The trailer brake control module will receive a serial data message from the BCM indicating that the vehicle rear fog lamps have been activated. The trailer brake control module will then apply system voltage to the trailer fog lamps through their respective voltage supply circuits.

Conditions for Running the DTC

When the trailer fog lamps are turned on.

Conditions for Setting the DTC

B3891 02

The trailer brake control module detects a short to ground on the trailer fog lamp control circuit.

B3891 04

The trailer brake control module detects an open/high resistance on the trailer fog lamp control circuit.

Action Taken When the DTC Sets

The trailer fog lamps will be inoperative.

Conditions for Clearing the DTC


• The DTC will be current for as long as the fault is present.

• When the fault is no longer present, the DTC will be a history DTC.

• A history DTC will clear after 40 ignition cycles.


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 scan tool information

Circuit/System Verification

Ignition ON, observe the function of the trailer fog lamps while commanding the Trailer Fog Lamp ON and OFF with a scan tool. The trailer fog lamps should turn ON and OFF while changing between the commanded states.

Circuit/System Testing


1. Ignition OFF, disconnect the X1 harness connector at the K68 trailer lighting control module.

2. Test for infinite resistance between the control circuit terminal 8 and ground.

3. If less than the specified value, test the control circuit for a short to ground. If the circuit tests normal, replace the X88 trailer connector.

4. Test for less than 1 Ω between the control circuit terminal 8 at the K68 trailer lighting control module and control circuit terminal 2 at the X88 trailer connector.

5. If greater than the specified range, test the control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the X88 trailer connector.

6. If all the circuits test normal, replace the K68 trailer lighting control module.


Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for trailer brake control module replacement, programming and setup