For 1990-2009 cars

### 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 Ground
DTC 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 5-Volt Reference | C0870 03 | C0870 03 | C0870 07 | --
Brake Pedal Position Sensor Signal | C0277 06 | C0277 06 | C0277 07 | C0277 09
Stop Lamp Relay Control | B3903 02 | B3903 05 | B3903 05 | --
Left Rear Tail/Stop Lamp Control | 1 | 1 | 1 | --
Right Rear Tail/Stop Lamp Control | 1 | 1 | 1 | --
Brake Pedal Position Sensor Low Reference | -- | C0277 06 | C0277 07 | --
Center High Mounted Stop Lamp (CHMSL) Ground | -- | 1 | -- | --
Left Rear Tail/Stop Lamp Ground | -- | 1 | -- | --
Right Rear Tail/Stop Lamp Ground | -- | 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 voltage to the stop lamp relay control circuit. This energizes the stop lamp relay coil causing the relay switch contacts to close. With the relay switch contacts closed, battery voltage flows from the STOP LAMP fuse through the relay switch contacts to the rear stop lamps, center high mounted stop lamp (CHMSL), 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 relay control circuit.

#### B3903 05

This DTC will set when the BCM detects a short to voltage or an open in the stop lamp relay control 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.

• | A history DTC will clear once 100 consecutive malfunction-free ignition cycles have occurred.

### Reference Information

#### Schematic Reference

Exterior Lights Schematics

#### Connector End View Reference

Component Connector End Views

#### Description and Operation

Exterior Lighting Systems Description and Operation

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Scan Tool Reference

Control Module References for scan tool information

### Circuit/System Testing

1. Ignition OFF, disconnect the stop lamp relay.
2. Ignition OFF, exterior lamps OFF, test for less than 5 ohms between the ground circuit terminal 86 and ground.

⇒ | If greater than the specified range, test the ground circuit for an open/high resistance.

3. Connect a test lamp between the control circuit terminal 85 and ground.
4. Ignition ON, command the CHMSL ON and OFF by applying and releasing the brake pedal. The test lamp should turn ON and OFF as the brake pedal is applied and released.

⇒ | If the test lamp is always OFF, test the control 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 control circuit for a short to voltage. If the circuit tests normal, replace the BCM.

5. If all circuits test normal, test or replace the stop lamp relay.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Relay Replacement

• | Control Module References for BCM replacement, setup, and programming