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 P0572: Brake Switch Circuit 1 Low Voltage
DTC P0573: Brake Switch Circuit 1 High Voltage

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
Stop Lamp Switch Signal | P0572 | P0572 | P0573 | P0571

### Circuit/System Description

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.

### Conditions for Running the DTC

• | The engine is ON.

• | Battery voltage is greater than 11.5 volts.

### Conditions for Setting the DTC

#### 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.

### Actions Taken When the DTC Sets

• | Stop lamp switch input is ignored.

• | The cruise control system is disabled.

### Conditions for Clearing the DTC

• | The condition responsible for setting the DTC no longer exists.

• | A history DTC will clear after 40 malfunction-free ignition cycles have occurred.

### Reference Information

#### Schematic Reference

• | Exterior Lights Schematics

• | Cruise Control Schematics

#### Connector End View Reference

Component Connector End Views

#### Description and Operation

Cruise Control 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 Verification

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

### Circuit/System Testing

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 2 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 5 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 3 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 5 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 X1 module (LNF)

• | ECM X1 and TCM X1 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 circuit tests normal, replace the TCM.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

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