The stop lamp switch signal circuit is a direct hardwire input to the engine control module (ECM), the transmission control module (TCM) and the center high mounted stop lamp (CHMSL). The modules monitor the stop lamp switch signal circuit to detect when the brake pedal has been applied. The BCM monitors the brake apply sensor signal circuit to determine when the brake pedal is applied. When the brake pedal is applied, the BCM sends a serial data message to the modules indicating that the brake pedal has been applied.

P0571

When the ECM detects that there is a 4.2 km/h (2.6 mph) or greater decrease in vehicle speed within 0.25 seconds without a transition of the stop lamp switch, the ECM sets DTC P0571.

P0572

The ECM detects a low voltage signal on the stop lamp switch signal circuit when the serial data message from the BCM indicates the brakes are applied.

P0573

The ECM detects a high voltage signal on the stop lamp switch signal circuit when the serial data message from the BCM indicates the brakes are not applied.

Schematic Reference

Connector End View Reference

Component Connector End Views

Description and Operation

Cruise Control Description and Operation

Electrical Information Reference

Scan Tool Reference

Control Module References for scan tool information

Inspect for proper operation of the stop lamps, including the CHMSL. Refer to Exterior Lighting Systems Description and Operation.

1. Ignition OFF, disconnect the CHMSL relay.
2. Ignition OFF, exterior lamps OFF, test for less than 5.0 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. Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal 87 and ground.

⇒	If the test lamp illuminates, test the control circuit for a short to voltage.

4. Verify that a test lamp illuminates between the B+ circuit terminal 30 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 control circuit terminal 87 for a short to ground. If the circuit tests normal, test or replace the CHMSL relay.

5. Ignition OFF, connect the CHMSL relay.
6. Ignition OFF, disconnect the harness connectors at the appropriate control modules listed below.

•	ECM X2 and TCM module (LNF)

•	ECM X1 and TCM module (L61 or LE5)

7. Connect a test lamp between the appropriate signal circuit terminal listed below and ground.

•	ECM harness connector X2 terminal 52 (LNF)

•	ECM harness connector X1 terminal 55 (L61 or LE5)

8. Ignition ON, press and release the brake pedal. Verify the test lamp turns ON and OFF when pressing and releasing the brake pedal.

⇒	If the test lamp is always ON, test the signal circuit for a short to voltage. If the circuit tests normal, replace the BCM.

⇒	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.

9. Ignition OFF, connect the harness connector at the ECM module.
10. Connect a test lamp between the appropriate signal circuit terminal listed below and ground.

•	TCM terminal 42 (LNF)

•	TCM terminal 42 (L61 or LE5)

11. Ignition ON, press and release the brake pedal. Verify the test lamp turns ON and OFF when pressing and releasing the brake pedal.

⇒	If the test lamp is always OFF, test the signal circuit for an open/high resistance. If the circuit test normal, replace the ECM module.

12. If all circuits test normal, replace the TCM.

Relay Test

1. Ignition OFF, disconnect the CHMSL relay.
2. Test for 60-180 ohms of resistance between terminals 85 and 86.

⇒	If the resistance is not within the specified range, replace the relay.

3. Test for infinite resistance between the following terminals:

•

30 and 86

•

30 and 87

•

30 and 85

•

85 and 87

⇒	If less than infinite resistance, replace the CHMSL relay.

4. Install a 20-amp fused jumper wire between relay terminal 85 and 12 volts. Install a jumper wire between relay terminal 86 and ground. Test for less than 2.0 ohms of resistance between terminals 30 and 87.

⇒	If greater than 2.0 ohms, replace the CHMSL relay.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for BCM, ECM or TCM replacement, setup, and programming