B2540

DTC B2540

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle (with HP2) (Initial Inspection and Diagnostic Overview)Diagnostic System Check - Vehicle (without HP2) (Initial Inspection and Diagnostic Overview) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Initial Inspection and Diagnostic Overview) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Initial Inspection and Diagnostic Overview) provides an overview of each diagnostic category.

DTC Descriptors
DTC B2540 02
- Rear Fog Lamp Control Circuit Short to Ground

DTC B2540 05
- Rear Fog Lamp Control 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 to the rear fog lamp control circuit to the rear fog lamp.

Conditions for Running the DTC

* Battery voltage must be between 9-16 volts.
* Rear fog lamps ON.

Conditions for Setting the DTC

B2540 02

DTC B2540 02 will set when the BCM detects a short to ground in the rear fog lamp switch signal circuit.

B2540 05

DTC B2540 05 will set when the BCM detects a short to voltage or an open/high resistance in the rear fog lamp switch signal circuit.

Action Taken When the DTC Sets

When DTC B2540 is present, the rear fog lamps will not operate when the park lamps are ON and the rear fog lamp switch is active.

Conditions for Clearing the DTC

* The condition responsible for setting the DTC no longer exists.
* A history DTC will clear once 100 consecutive malfunction-free ignition cycles have occurred.

Reference Information
Schematic Reference

Fog Lights Schematics (Fog Lights Schematics)
Connector End View Reference

Component Connector End Views (Connector Views)
Description and Operation

Exterior Lighting Systems Description and Operation (Exterior Lighting Systems Description and Operation)
Electrical Information Reference

* Circuit Testing (Component Tests and General Diagnostics)
* Connector Repairs (Component Tests and General Diagnostics)
* Testing for Intermittent Conditions and Poor Connections (Component Tests and General Diagnostics)
* Wiring Repairs (Component Tests and General Diagnostics)

Scan Tool Reference

Control Module References (Programming and Relearning) for scan tool information

Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the headlamp switch.
2. Verify that a test lamp illuminates between B+ and the ground circuit terminal 8.

• If the test lamp does not illuminate, test the ground circuit for an open/high resistance.

3. Ignition ON, verify the scan tool Rear Fog Lamp Switch parameter is INACTIVE.

• If not the specified value, test the signal circuit terminal 6 for a short to ground. If the circuit tests normal, replace the XBCM.

4. Install a 3A fused jumper wire between the signal circuit terminal 6 and ground. Verify the scan tool Rear Fog Lamp Switch parameter is ACTIVE.

• If not the specified value, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the XBCM.

5. If all circuits test normal, test or replace the headlamp switch.

Repair Instructions

Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.

* Rear Fog Lamp Bulb Replacement (Cadillac Export) ()
* Control Module References (Programming and Relearning) for XBCM replacement, setup, and programming