B3903

DTC B3903

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 B3903 02
- Stoplamp Relay Circuit Short to Ground

DTC B3903 05
- Stoplamp Relay Circuit Short to Battery or Open




Circuit/System 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.


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.
* The CHMSL output is disabled.



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.



Diagnostic Aids

* An internal short in the CHMSL may cause this code to set.
* A faulty brake lamp diagnostic resistor may cause B3903 05 to set.



Reference Information
Schematic Reference

Exterior Lights Schematics (Exterior Lights Diagrams)
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. Disconnect the CHMSL.
2. Ignition ON, verify that a test lamp illuminates between the ground circuit terminal B and B+.

• If the test lamp does not illuminate, test the ground circuit for a short to voltage or an open/high resistance.

3. Verify that a test lamp does not illuminate between the stop lamp switch signal circuit terminal A and ground.

• If the test lamp illuminates, test the stop lamp switch signal circuit for a short to voltage.

4. Test the stop lamp switch signal circuit for a short to ground or an open/high resistance.
5. If all circuits test normal, replace the BCM.



Repair Instructions

Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.


Control Module References (Programming and Relearning) for BCM replacement, setup, and programming