GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2004 🢒 XLR 🢒 Vehicle Control Systems 🢒 Vehicle DTC Information 🢒 DTC B2540

Circuit Description

The rear fog lamp switch signal circuit is grounded momentarily by pressing the rear fog lamp switch. The BCM energizes the rear fog lamp relay by applying battery positive voltage to the rear fog lamp relay control circuit. When the rear fog lamp relay is energized, battery positive voltage is applied to the rear fog lamps supply voltage circuit which illuminates the rear fog lamp.

Conditions for Running the DTC

The battery voltage must be between 9.0-16.0 volts.

Conditions for Setting the DTC
    • DTC B2540 02 The rear fog lamp relay control circuit is shorted to ground.
    • DTC B2540 05 The rear fog lamp relay control circuit is shorted to battery positive voltage or open.

Action Taken When the DTC Sets
    • DTC B2540 02 rear fog lamps will not operate.
    • DTC B2540 05 rear fog lamps will be on at all times or will not operate.

Conditions for Clearing the DTC
    •  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.
    • History and current DTCs can be cleared using a scan tool.

Step

Action

Yes

No

Schematic Reference: Fog Lights Schematics

Connector End View Reference: Master Electrical Component List in Wiring Systems

1

Did you perform the Diagnostic System Check?

Go to Step 2

Go to Diagnostic System Check - Vehicle in Vehicle DTC Information

2

  1. Turn ON the ignition.
  2. Turn ON the park lamps
  3. Turn ON the rear fog lamps.
  4. Turn OFF the rear fog lamps.

Do the rear fog lamps turn ON and OFF?

Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems

Go to Step 3

3

Is DTC B2540 02 present?

Go to Step 4

Go to Step 6

4

  1. Remove the rear fog lamp relay from the I/P relay center.
  2. Connect a test lamp between battery positive voltage and the rear fog lamp relay control circuit at the I/P relay center.

Does the test lamp illuminate?

Go to Step 5

Go to Step 9

5

Test for a short to ground in the rear fog lamp relay signal circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 11

Go to Step 10

6

  1. Disconnect the BCM Connector C1.
  2. Connect a test lamp between the rear fog lamp relay control circuit and ground.

Does the test lamp illuminate?

Go to Step 7

Go to Step 8

7

Test for a short to battery positive voltage in the rear fog lamp switch signal circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 11

Go to Step 9

8

Test for a poor connection or an open in the rear fog lamp relay control circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 11

Go to Step 10

9

Replace the rear fog lamp relay. Refer to Relay Replacement in Wiring Systems.

Did you complete the replacement?

Go to Step 10

--

10

Important: After replacing the BCM perform the setup procedure. Refer to Body Control Module (BCM) Programming/RPO Configuration in Computer/Integrating Systems.

Replace the BCM. Refer to Body Control Module Replacement in Computer/Integrating Systems.

Did you complete the replacement?

Go to Step 10

--

11
  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