B2540
DTC B2540
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle (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 Descriptor
DTC B2540
- Rear Fog Lamps Control Circuit
Diagnostic Fault Information
Circuit/System Description
The rear fog lamp relay is supplied with battery voltage at all times from the REAR FOG fuse located in the rear fuse block. The rear fog lamp switch signal circuit is grounded momentarily by pressing the rear fog lamp switch. The body control module (BCM) energizes the rear fog lamp relay by applying ground to the rear fog lamp relay control circuit. When the rear fog lamp relay is energized, the relay switch contacts close and battery voltage is applied through the REAR FOG fuse to the rear fog lamp supply voltage circuit which illuminates the rear fog lamps.
Conditions for Running the DTC
The battery voltage must be between 9-16 volts.
Conditions for Setting the DTC
* When the BCM receives a rear fog lamp switch OFF command and the rear fog lamp relay control circuit is shorted to ground.
* When the BCM receives a rear fog lamp switch ON command and the rear fog lamp relay control circuit is shorted to voltage or open.
Action Taken When the DTC Sets
The rear fog lamps will be inoperative or always ON.
Conditions for Clearing the DTC
* This DTC will clear on current status after the condition for setting the fault is corrected.
* A history DTC will clear after 100 consecutive ignition cycles without a fault present.
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 rear fog lamp relay.
2. Ignition ON, verify that a test lamp does not illuminate between the rear fog lamp control circuit terminal 87 and ground.
• If the test lamp illuminates, test the control circuit for a short to voltage.
3. Verify that a test lamp illuminates between the B+ circuit terminal 30 and ground.
• If the test lamp does not illuminate, test the B+ circuit terminal for a short to ground or an open/high resistance.
4. Verify that a test lamp illuminates between the B+ circuit terminal 85 and ground.
• If the test lamp does not illuminate, test the B+ circuit terminal for a short to ground or an open/high resistance.
5. Disconnect the harness connector at the inoperative rear fog lamp.
6. Test for less than 5.0 ohms between the rear fog lamp ground circuit terminal E and ground.
• If greater than the specified range, test the ground circuit for an open/high resistance.
7. Ignition OFF, connect a 10A fused jumper wire between the B+ circuit terminal 30 and the control circuit terminal 87..
8. Connect a test lamp between the control circuit terminal C and ground. Verify the test lamp illuminates.
• If the test lamp is always OFF, test the control circuit for a short to ground or an open/high resistance.
• If the test lamp is always ON, test the control circuit for a short to voltage.
9. If all circuits test normal, replace the rear fog lamp relay.
Repair Instructions
Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.
Control Module References (Programming and Relearning) for BCM replacement, programming, and setup.