Stop Lamps Malfunction

Stop Lamps Malfunction

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle (Diagnostic System Check - Vehicle) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Strategy Based Diagnosis) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Diagnostic Procedure Instructions) provides an overview of each diagnostic category.



Diagnostic Fault Information






Circuit/System Description

B+ is supplied to the STOP lamp fuse and Stop Lamp Switch at all times. When the driver presses the brake pedal, the switch contacts close and B+ is supplied to the body control module (BCM), electronic brake control module (EBCM), powertrain control module (PCM), and the Center High Mounted Stop Lamp (CHMSL). Ground is supplied at all times to the Tail/Stop Lamp- Right at G340 and at the Tail/Stop Lamp-Left and CHMSL at G350.


Reference Information
Schematic Reference

Exterior Lights Schematics (Exterior Lights Schematics)
Connector End View Reference

Component Connector End Views (Component Connector End Views)
Description and Operation

Exterior Lighting Systems Description and Operation (Exterior Lighting Systems Description and Operation)
Electrical Information Reference

* Circuit Testing (Component Tests and General Diagnostics)
* Connector Repairs (Component Tests and General Diagnostics)
* Testing for Intermittent Conditions and Poor Connections (Component Tests and General Diagnostics)
* Wiring Repairs (Component Tests and General Diagnostics)

Scan Tool Reference

Control Module References (Programming and Relearning) for Scan Tool Information


Circuit/System Verification

1. Ignition ON, observe the left and right tail/stop lamps while pressing and releasing the brake pedal. The tail/stop lamps should turn on and off.

• If not refer to the stop lamp switch circuit test.

2. Observe the center high mount stop lamp while pressing and releasing the brake pedal. The lamp should turn on and off.

• If not refer to the center high mount stop lamp circuit test.




Circuit/System Testing


Stop Lamp Switch Circuit Test
1. Ignition OFF, disconnect the harness connector at the stop lamp switch.
2. Ignition ON, verify that a test lamp illuminates between the B+ circuit terminal 1 and ground.

• If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal, and the B+ circuit fuse is open, test the signal circuit terminal 2 for a short to ground.

3. Connect the harness connector at the stop lamp switch.
4. Disconnect the harness connector X1 at the BCM.
5. Verify that a test lamp does not illuminate between the stop lamp switch signal circuit terminal A37 and ground.

• If the test lamp illuminates, test the signal circuit for a short to voltage.

6. Verify that the test lamp illuminates when the brake pedal is pressed.

• If the test lamp does not illuminate, test the signal circuit terminal A37 for an open/high resistance. If the circuit tests normal, replace the stop lamp switch.

7. If all circuits test normal, replace the BCM.



Center High Mount Stop Lamp Circuit Test
1. Ignition OFF, disconnect the harness connector at the CHMSL.
2. Test for less than 1.0 ohm between the ground circuit terminal 1 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 2 and ground.
4. Ignition ON, apply and release 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 ON, test the control circuit for a short to voltage.


• If the test lamp is always OFF, test the control circuit for a short to ground or an open/high resistance.

5. If all circuits test normal, replace the CHMSL.



Repair Instructions

Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.

* Stop Lamp Switch Replacement (Service and Repair)
* High Mount Stop Lamp Replacement (Service and Repair)
* Control Module References (Programming and Relearning) for BCM replacement, setup, and programming.