B3903

DTC B3903

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. Initial Inspection and Diagnostic Overview
* Review Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions 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.

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 after completing the diagnostic procedure. Verification Tests