GM Service Manual Online
For 1990-2009 cars only

Circuit Description

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 battery voltage to the rear fog lamp relay control circuit. When the rear fog lamp relay is energized, battery voltage is applied via the REAR FOG/ALDL TOP SW fuse to the rear fog lamp supply voltage circuit which illuminates the rear fog lamps.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC B2540 Rear Fog Lamps Control Circuit

DTC Symptom

DTC Symptom Descriptor

02

Short to Ground

05

Open or Short to Voltage

Conditions for Running the DTC

Battery voltage must be between 9-16 volts.

Conditions for Setting the DTC

    • DTC B2540 02 will set when the BCM detects a short to ground in the rear fog lamp relay control circuit.
    • DTC B2540 05 will set when the BCM detects an open or short to voltage in the rear fog lamp relay control circuit.

Action Taken When the DTC Sets

    • When DTC B2540 02 is present, the rear fog lamps will not operate when the park lamps are ON and the rear fog lamp switch is active.
    • When DTC B2540 05 is present, the rear fog lamps and/or the rear fog lamp indicator will be ON at all times or will not operate.

Conditions for Clearing the DTC

    • The condition responsible for setting the DTC no longer exists.
    • You issue a scan tool CLEAR DTCs command.
    • A history DTC will clear once 100 consecutive malfunction-free ignition cycles have occurred.

Diagnostic Aids

Thoroughly inspect the wiring and connectors. An incomplete inspection of the wiring and connectors may result in misdiagnosis causing part replacement with the reappearance of the malfunction. If an intermittent malfunction exists, refer to Testing for Intermittent Conditions and Poor Connections .

Step

Action

Yes

No

Schematic Reference: Fog Lights Schematics

Connector End View Reference: Master Electrical Component List

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.

Is DTC B2540 02 present?

Go to Step 3

Go to Step 5

3

  1. Remove the rear fog lamp relay from the instrument panel (I/P) relay block.
  2. Connect a test lamp between the rear fog lamp relay control circuit and battery voltage at the I/P relay block.
  3. Turn ON the ignition, with the engine OFF.

Does the test lamp illuminate?

Go to Step 4

Go to Step 7

4

Locate and repair a short to ground in the rear fog lamp relay control circuit. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 10

Go to Step 8

5

  1. Disconnect the body control module (BCM) C2 harness connector.
  2. Connect a test lamp between the rear fog lamp relay control circuit and ground.

Does the test lamp illuminate?

Go to Step 6

Go to Step 8

6

Locate and repair an open or short to battery voltage in the rear fog lamp relay control circuit. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 10

Go to Step 7

7

Replace the rear fog lamp relay. Refer to Relay Replacement .

Did you complete the replacement?

Go to Step 10

--

8

Inspect for poor connections at the C2 harness connector of the BCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Step 10

Go to Step 9

9

Replace the BCM. Refer to Control Module References .

Did you complete the replacement?

Go to Step 10

--

10

  1. Use a scan tool in order to clear the DTCs.
  2. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC reset?

Go to Step 2

System OK