• 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.

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 V 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 left and right stop lamp control circuits as well as the center high mounted stop lamp control circuit illuminating the left and right stop lamps and the center high mounted stop lamp.

• X88 = Chevrolet
• Z88 = GMC

• Battery voltage must be between 9–16 V.
• Brakes APPLIED.

The BCM detects a short to voltage or an open/high resistance in the left stop lamp control circuit.

The BCM detects a short to ground in the left stop lamp control circuit.

The BCM detects a short to voltage or an open/high resistance in the right stop lamp control circuit.

The BCM detects a short to ground in the right stop lamp control circuit.

The appropriate stop lamp will not operate.

• The condition responsible for setting the DTC no longer exists.
• A history DTC will clear once 100 consecutive malfunction-free ignition cycles have occurred.

Exterior Lights Schematics

Master Electrical Component List

Exterior Lighting Systems Description and Operation

• Circuit Testing
• Connector Repairs
• Testing for Intermittent Conditions and Poor Connections
• Wiring Repairs

Control Module References for scan tool information

1. Ignition OFF, exterior lamps OFF, disconnect the harness connector at the E42L Tail Lamp Assembly – Left.
2. Test for less than 5 Ω between the appropriate ground circuit terminal listed below and ground.
• {X88 without UGE} Terminal 3
• {X88 with UGE} Terminal 4
• {Z88} Terminal 4
• If 5 Ω or greater
1. Ignition OFF.
2. Test for less than 2 Ω in the ground circuit end to end.
• If 2 Ω or greater, repair the open/high resistance in the circuit.
• If less than 2 Ω, repair the open/high resistance in the ground connection.
• If less than 5 Ω
3. Connect a test lamp between the appropriate control circuit terminal listed below and ground, ignition ON.
• {X88} Terminal 1
• {Z88} Terminal 3
4. Verify the test lamp turns ON and OFF when commanding the Left Stop Lamp ON and OFF with a scan tool.
• If the test lamp is always OFF
1. Ignition OFF, disconnect the X4 harness connector at the K9 Body Control Module.
2. Test for infinite resistance between the control circuit and ground.
• If less than infinite resistance, repair the short to ground on the circuit.
• If infinite resistance
3. Test for less than 2 Ω in the control circuit end to end.
• If 2 Ω or greater, repair the open/high resistance in the circuit.
• If less than 2 Ω, replace the K9 Body Control Module.
• If the test lamp is always ON
1. Ignition OFF, disconnect the X4 harness connector at the K9 Body Control Module, ignition ON.
2. Test for less than 1 V between the control circuit terminal and ground.
• If 1 V or greater, repair the short to voltage on the circuit.
• If less than 1 V, replace the K9 Body Control Module.
• If the test lamp turns ON and OFF
5. Test or replace the left stop lamp.

1. Ignition OFF, exterior lamps OFF, disconnect the harness connector at the E42R Tail Lamp Assembly – Right.
2. Test for less than 5 Ω between the appropriate ground circuit terminal listed below and ground.
• {X88 without UGE} Terminal 3
• {X88 with UGE} Terminal 4
• {Z88} Terminal 4
• If 5 Ω or greater
1. Ignition OFF.
2. Test for less than 2 Ω in the ground circuit end to end.
• If 2 Ω or greater, repair the open/high resistance in the circuit.
• If less than 2 Ω, repair the open/high resistance in the ground connection.
• If less than 5 Ω
3. Connect a test lamp between the appropriate control circuit terminal listed below and ground, ignition ON.
• {X88} Terminal 1
• {Z88} Terminal 3
4. Verify the test lamp turns ON and OFF when commanding the Right Stop Lamp ON and OFF with a scan tool.
• If the test lamp is always OFF
1. Ignition OFF, disconnect the X5 harness connector at the K9 Body Control Module.
2. Test for infinite resistance between the control circuit and ground.
• If less than infinite resistance, repair the short to ground on the circuit.
• If infinite resistance
3. Test for less than 2 Ω in the control circuit end to end.
• If 2 Ω or greater, repair the open/high resistance in the circuit.
• If less than 2 Ω, replace the K9 Body Control Module.
• If the test lamp is always ON
1. Ignition OFF, disconnect the X5 harness connector at the K9 Body Control Module, ignition ON.
2. Test for less than 1 V between the control circuit terminal and ground.
• If 1 V or greater, repair the short to voltage on the circuit.
• If less than 1 V, replace the K9 Body Control Module.
• If the test lamp turns ON and OFF
5. Test or replace the right stop lamp.

Perform the Diagnostic Repair Verification after completing the repair.

• Rear Body Structure Stop Lamp Replacement
• Control Module References for BCM replacement, programming, and setup