Diagnostic Instructions
| • | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. |
| • | Review Strategy Based Diagnosis for an overview of the diagnostic approach. |
| • | Diagnostic Procedure Instructions provides an overview of each diagnostic category. |
DTC Descriptors
DTC B3903 02: Stop Lamp Relay Circuit Short to GroundDTC B3903 05: Stop Lamp Relay Circuit Open or Short to Voltage
Diagnostic Fault Information
Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance |
|---|---|---|---|---|
Brake Pedal Position Sensor Signal | C0277 06 | C0277 06 | C0277 07 | C0277 09 |
Brake Pedal Position Sensor 5-Volt Reference | C0870 03 | C0870 03 | C0870 07 | -- |
Brake Pedal Position Sensor Low Reference | -- | C0277 06 | C0277 07 | -- |
Stop Lamp Switch Signal | B3903 02, P0572 | B3903 05, P0573 | B3903 05, P0573 | -- |
Left Rear Stop/Turn Lamp Supply Voltage | 1 | 1 | 1 | -- |
Right Rear Stop/Turn Lamp Supply Voltage | 1 | 1 | 1 | -- |
Center High Mounted Stop Lamp (CHMSL) Ground | -- | 1 | 1 | -- |
Left Rear Stop/Turn Lamp Ground | -- | 1 | 1 | -- |
Right Rear Stop/Turn Lamp Ground | -- | 1 | 1 | -- |
1. Stop Lamps Malfunction | ||||
Circuit/System Description
The brake pedal position sensor is used to sense the action of the driver application of the brake pedal. The brake pedal position sensor provides an analog voltage signal that will increase as the brake pedal is applied. The body control module (BCM) provides a low reference signal and a 5-volt reference voltage to the brake pedal position sensor. When the variable signal reaches a voltage threshold indicating the brakes have been applied, the BCM will apply battery positive voltage to the stop lamps, center high mounted stop lamp (CHMSL), transmission control module (TCM), and engine control module (ECM). The stop lamps will not operate unless the ignition is in the ON position.
Conditions for Running the DTC
Battery voltage must be between 9-16 volts.
Conditions for Setting the DTC
B3903 02
This DTC will set when the BCM detects a short to ground in the stop lamp switch signal circuit.
B3903 05
This DTC will set when the BCM detects an open or short to voltage in the stop lamp switch signal circuit.
Action Taken When the DTC Sets
When DTC B3903 is present, the service vehicle soon indicator will illuminate.
Conditions for Clearing the DTC
| • | The condition responsible for setting the DTC no longer exists. |
| • | You issue a scan tool CLEAR DTCs command. |
| • | A history DTC will clear once 100 consecutive malfunction-free ignition cycles have occurred. |
Diagnostic Aids
Important: The stop lamps will not illuminate unless the ignition is in the ON position. For more detailed information concerning stop lamp operation, refer to Exterior Lighting Systems Description and Operation .
Reference Information
Schematic Reference
Connector End View Reference
Lighting Systems Connector End Views
Electrical Information Reference
Scan Tool Reference
Circuit/System Testing
- Ignition OFF, disconnect the harness connector at the CHMSL.
- Connect a test lamp between the stop lamp switch signal circuit terminal A and ground.
- Ignition ON, apply the brake pedal ON and OFF. The test lamp should turn ON and OFF as the brake pedal is applied and released.
- If all circuits test normal, replace the CHMSL.
| ⇒ | If the test lamp is always OFF, test the signal circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the BCM. |
| ⇒ | If the test lamp is always ON, test the signal circuit for a short to voltage. If the circuit tests normal, replace the BCM. |
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
| • | Control Module References for BCM replacement, setup, and programming |
