• 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

• Exterior Lights Schematics
• Body Control System 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 ON.
2. Perform the brake pedal position sensor learn procedure for the K9 Body Control Module. Refer to Brake Pedal Position Sensor Calibration.
3. Verify the scan tool Brake Pedal Applied parameter changes between Active and Inactive while pressing and releasing the brake pedal.
• If the parameter does not change

  Refer to Brake Pedal Position Sensor Malfunction.

• If the parameter changes
4. Verify the center high mounted stop lamp turns ON and OFF when commanding the Center Stop Lamp ON and OFF with a scan tool.
• If the center high mounted stop does not turn ON and OFF

  Refer to Center High Mount Stop Lamp Malfunction.

• If the center high mounted stop turns ON and OFF
5. Verify the left stop lamp turns ON and OFF when commanding the Left Stop Lamp ON and OFF with a scan tool.
• If the left stop lamp does not turn ON and OFF

  Refer to Stop Lamps Malfunction

• If the left stop lamp turns ON and OFF
6. Verify the right stop lamp turns ON and OFF when commanding the Right Stop Lamp ON and OFF with a scan tool.
• If the right stop lamp does not turn ON and OFF

  Refer to Stop Lamps Malfunction.

• If the right stop lamp turns ON and OFF
7. All OK.

1. Ignition OFF, disconnect the harness connector at the B22 Brake Pedal Position Sensor, ignition ON.
2. Verify a test lamp illuminates between the low reference circuit terminal 2 and B+.
• If the test lamp does not illuminate
1. Ignition OFF, disconnect the X1 harness connector at the K9 Body Control Module.
2. Test for less than 2 Ω in the low reference 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 illuminates
3. Test for less than 1 V between the low reference circuit terminal 2 and ground.
• If 1 V or greater
1. Ignition OFF, disconnect the X1 harness connector at the K9 Body Control Module, ignition ON.
2. Test for less than 1 V between the low reference circuit 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 less than 1 V
4. Test for 4.8–5.2 V between the 5 V reference circuit terminal 1 and ground.
• If less than 4.8 V
1. Ignition OFF, disconnect the X2 harness connector at the K9 Body Control Module.
2. Test for infinite resistance between the 5 V reference 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 5 V reference 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 greater than 5.2 V
1. Ignition OFF, disconnect the X2 harness connector at the K9 Body Control Module, ignition ON.
2. Test for less than 1 V between the 5 V reference circuit 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 between 4.8–5.2 V
5. Verify the scan tool Brake Pedal Applied parameter is Inactive.
• If not Inactive
1. Ignition OFF, disconnect the X2 harness connector at the K9 Body Control Module, ignition ON.
2. Test for less than 1 V between the signal circuit terminal 3 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 Inactive
6. Install a 3 A fused jumper wire between the 5 V reference circuit terminal 1 and signal circuit terminal 3.
7. Verify the scan tool Brake Pedal Applied parameter is Active.
• If not Active
1. Ignition OFF, disconnect the X2 harness connector at the K9 Body Control Module.
2. Test for infinite resistance between the signal 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 signal 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 Active
8. Connect the harness connector at the B22 Brake Pedal Position Sensor.
9. Perform the brake pedal position sensor learn procedure for the K9 Body Control Module. Refer to Brake Pedal Position Sensor Calibration.
10. Verify the scan tool Brake Pedal Applied parameter changes between Active and Inactive while pressing and releasing the brake pedal.
• If the parameter does not change

  Test or replace the B22 Brake Pedal Position Sensor.

• If the parameter changes

  All OK

1. Ignition OFF, exterior lamps OFF, disconnect the harness connector at the appropriate E42 Tail Lamp Assembly.
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 appropriate Stop Lamp ON and OFF with a scan tool.
• If the test lamp is always OFF
1. Ignition OFF, disconnect the appropriate harness connector listed below at the K9 Body Control Module.
• X4 – left stop lamps
• X5 – right stop lamps
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 appropriate harness connector listed below at the K9 Body Control Module.
• X4 – left stop lamps
• X5 – right stop lamps
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 appropriate stop lamp.

1. Ignition OFF, exterior lamps OFF, disconnect the harness connector at the E6 Center High Mounted Stop Lamp.
2. Test for less than 10 Ω between the ground circuit terminal 1 and ground.
• If 10 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 10 Ω
3. Connect a test lamp between the control circuit terminal 2 and ground, ignition ON.
4. Verify the test lamp turns ON and OFF when commanding the Center 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 E6 Center High Mounted Stop Lamp.

Perform the Diagnostic Repair Verification after completing the repair.

• High Mount Stop Lamp Replacement
• Brake Pedal Position Sensor Replacement
• Brake Pedal Position Sensor Calibration
• Rear Body Structure Stop Lamp Replacement
• Stop Bulb Replacement
• Rear Combination Lamp Replacement
• Control Module References for BCM replacement, programming, and setup