Test Description

WARNING: This page is about a different car, the 2002 Oldsmobile Bravada, 2002 GMC Envoy XL, 2002 GMC Envoy, and 2002 Chevrolet TrailBlazer. However, it is still accessible from the selected car via links, so may be relevant.

The numbers below refer to the step numbers on the diagnostic table.

2: Listen for an audible click when the rear fog lamp relay operates. Command both the ON and OFF states. Repeat the commands as necessary.
3: Tests for voltage at the coil side of the rear fog lamp relay.
4: Verifies that the body control module (BCM) is providing ground to the rear fog lamp relay.
5: Tests if ground is constantly being applied to the rear fog lamp relay.
6: Tests if there is opposing voltage present or an open in the rear fog lamp relay control circuit.
12: After replacement of the BCM you must calibrate the new module for proper operation.

DTC B2540

Step	Action	Yes	No
Schematic Reference: Fog Lights Schematics
1	Did you perform the Lighting Systems Diagnostic System Check?	Go to Step 2	Go to Diagnostic System Check - Lighting Systems
2	Install a scan tool. Turn ON the ignition, with the engine OFF. With a scan tool, command the rear fog lamp relay ON and OFF. Does the rear fog lamp relay turn ON and OFF with each command?	Go to TESTING FOR INTERMITTENT CONDITIONS AND POOR CONNECTIONS in Wiring Systems	Go to Step 3
3	Turn OFF the ignition. Disconnect the rear fog lamp relay. Turn ON the ignition, with the engine OFF. Probe the battery positive voltage circuit of the rear fog lamp relay with a test lamp that is connected to a good ground. Does the test lamp illuminate?	Go to Step 4	Go to Step 10
4	Connect a test lamp between the control circuit of the high beam relay and the battery positive voltage circuit of the rear fog lamp relay. With a scan tool, command the rear fog lamp relay ON and OFF. Does the test lamp turn ON and OFF with each command?	Go to Step 8	Go to Step 5
5	Does the test lamp remain illuminated with each command?	Go to Step 7	Go to Step 6
6	Test the relay control circuit of the rear fog lamp relay and the indicator control circuit for a short to voltage or an open. Refer to CIRCUIT TESTING and WIRING REPAIRS in Wiring Systems. Did you find and correct the condition?	Go to Step 13	Go to Step 9
7	Test the relay control circuit of the rear fog lamp relay and the indicator control circuit for a short to ground. Refer to CIRCUIT TESTING and WIRING REPAIRS in Wiring Systems. Did you find and correct the condition?	Go to Step 13	Go to Step 9
8	Inspect for poor connections at the rear fog lamp relay. Refer to CIRCUIT TESTING and WIRING REPAIRS in Wiring Systems. Did you find and correct the condition?	Go to Step 13	Go to Step 11
9	Inspect for poor connections at the harness connector of the BCM. Refer to CIRCUIT TESTING and WIRING REPAIRS in Wiring Systems. Did you find and correct the condition?	Go to Step 13	Go to Step 12
10	Repair the battery positive voltage circuit of the rear fog lamp relay. Refer to WIRING REPAIRS in Wiring Systems. Did you complete the repair?	Go to Step 13	-
11	Replace the rear fog lamp relay. Refer to Fog Lamp Relay Replacement (RPO T96) Fog Lamp Relay Replacement (RPO T79) . Did you complete the replacement?	Go to Step 13	-
12	IMPORTANT: Perform the setup procedure for the BCM. Replace the body control module (BCM). Refer to BODY CONTROL MODULE REPLACEMENT in Body Control System.Did you complete the replacement?	Go to Step 13	-
13	Use the scan tool in order to clear the DTCs. 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