Diagnostic Instructions

1. Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
2. Review Strategy Based Diagnosis for an overview of the diagnostic approach.
3. Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptor

DTC B2540 01: Rear Fog Lamps Control Circuit Short to Battery

DTC B2540 02: Rear Fog Lamps Control Circuit Short to Ground

DTC B2540 04: Rear Fog Lamps Control Circuit Open

Diagnostic Fault Information

Circuit	Short to Ground	Open/High Resistance	Short to Voltage	Signal Performance
Rear Fog Lamps Control	B2540 02	B2540 04	B2540 01	--

Circuit/System Description

The body control module (BCM) energizes the rear fog lamp by applying voltage to the rear fog lamp control circuit. When the rear fog lamp control circuit is energized the rear fog lamp is illuminated.

Conditions for Running the DTC

Rear fog lamp activated

Conditions for Setting the DTC

B2540 01

The BCM detects a short to voltage on the control circuit.

B2540 02

The BCM detects a short to ground on the control circuit.

B2540 04

The BCM detects an open control circuit.

Conditions for Clearing the DTC

The condition for setting the DTC is no longer present.

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

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification

Ignition ON, perform the Rear Fog Lamps Relay Command test. The rear fog lamp should be ON/OFF when changing between the commanded states.

Circuit/System Testing

1. Ignition OFF, disconnect the appropriate harness connector at the tail lamp listed below:

•	E42L Tail lamp left

•	E42R Tail lamp right

2. Test for less than 5 Ω between the appropriate ground circuit harness connector terminal listed below and ground:

•	E42L Tail lamp left harness connector terminal F

•	E42R Tail lamp right harness connector terminal A

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

3. Connect a test lamp between the appropriate control circuit harness connector terminal listed below and ground :

•	E42L Tail lamp left harness connector terminal D

•	E42R Tail lamp right harness connector terminal C

4. Ignition ON, perform the Rear Fog Lamps Relay Command test. The rear fog lamp should be ON/OFF when changing between the commanded states.

⇒	If the test lamp is always ON, test the control circuit for a short to voltage. If the circuit tests normal, replace the K9 Body Control Module (BCM).

⇒	If the test lamp is always OFF, test the control circuit for a short to ground or an open/high resistance. If the circuits test normal, replace the K9 Body Control Module (BCM).

5. If all circuits test normal, test or replace the appropriate tail lamp.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.