GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2005 🢒 Cobalt 🢒 Body 🢒 Lighting Systems 🢒 DTC B3903

Circuit Description

The body control module (BCM) supplies voltage to the center high mount stop lamp (CHMSL) and the powertrain control module (PCM) when the stop lamps are active.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC B3903 Stop Lamp Relay Circuit

Conditions for Running the DTC
    • The BCM must be powered.
    • This test is run continuously.

Conditions for Setting the DTC

B3903 02

Will set if the stop lamp switch signal circuit is shorted to ground for 125 ms.

B3903 05

Will set if the stop lamp switch signal circuit is open or shorted to voltage for 125 ms.

Action Taken When the DTC Sets

When DTC B3903 is set the service vehicle soon lamp will illuminate.

Conditions for Clearing the DTC
    • The current DTC will clear when the fault is no longer present.
    • The ignition key must be cycled OFF and ON.
    • A history DTC clears after 100 consecutive ignition cycles in which no faults have occurred.
    • Clear the DTC with a scan tool.

Step

Action

Yes

No

Schematic Reference: Exterior 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.
  3. Retrieve DTCs.

Is the DTC retrieved as a current DTC?

Go to Step 3

Go to Testing for Intermittent Conditions and Poor Connections

3

Is DTC B3903 02 set?

Go to Step 5

Go to Step 4

4
  1. Place the ignition in the ON position.
  2. Ensure that the brake pedal is not applied.
  3. Observe the center high mount stop lamp (CHMSL).

Is the CHMSL illuminated?

Go to Step 6

Go to Step 7

5

Test the stop lamp switch signal circuit for a short to ground. Refer to Circuit Protection - Circuit Breakers and Wiring Repairs .

Did you find and correct the condition?

Go to Step 10

Go to Step 8

6

Test the stop lamp switch signal circuit for a short to voltage. Refer to Circuit Protection - Circuit Breakers and Wiring Repairs .

Did you find and correct the condition?

Go to Step 10

Go to Step 8

7

Test the stop lamp switch signal circuit for an open or high resistance. Refer to Circuit Protection - Circuit Breakers and Wiring Repairs .

Did you find and correct the condition?

Go to Step 10

Go to Step 8

8

Inspect for poor connections at the harness connector of the body control module (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 for replacement, setup, and programming.

Did you complete the replacement?

Go to Step 10

--

10
  1. Use the scan tool in order to clear the DTCs.
  2. Operate the vehicle within the Conditions for Running the DTC.

Does the DTC reset?

Go to Step 2

System OK