P1572

What Does Code P1572 Mean?

The diagnostic trouble code P1572 indicates that the Powertrain Control Module (PCM), also known as the Engine Control Module (ECM) in some vehicles, has detected an electrical malfunction within the brake pedal switch circuit. This switch, often referred to as the brake light switch or stop lamp switch, provides a critical input signal to the PCM and other control modules (e.g., Body Control Module, Transmission Control Module) indicating whether the driver is pressing the brake pedal. The PCM monitors the voltage signal from this circuit to confirm the brake pedal’s position – typically an open or closed state – corresponding to “brakes applied” or “brakes released.” A P1572 code is set when the PCM observes a signal that is inconsistent with expected parameters, such as a voltage reading that is continuously high or low, intermittent, or outside the specified operating range, suggesting an open circuit, a short to voltage, or a short to ground within the circuit. This anomaly prevents the PCM from accurately determining the driver’s braking intent, affecting various vehicle systems reliant on this input.

Common Symptoms

• Brake Lights Constantly On or Off: The most direct and common symptom, indicating a failure to illuminate or extinguish the rear brake lights.
• Cruise Control Malfunction: Inability to engage the cruise control system, or the cruise control failing to disengage when the brake pedal is pressed.
• Shift Interlock Issues: Difficulty shifting an automatic transmission out of the “Park” position, or conversely, being able to shift out of “Park” without pressing the brake pedal.
• ABS/Traction Control System Warning Lights: Illumination of the Anti-lock Braking System (ABS) or Traction Control System (TCS) warning lights due to an unreliable brake pedal input.
• Engine No-Start Condition: In vehicles equipped with a push-button start system or requiring brake pedal input to crank the engine, the vehicle may fail to start.
• Reduced Engine Power or Limp Mode: In some advanced vehicle systems, a critical brake signal malfunction can trigger a safety mode, leading to reduced engine performance.

What Causes the Code P1572?

• Faulty Brake Pedal Position Switch: The most prevalent cause, where the internal contacts of the switch become worn, corroded, or mechanically fail, leading to an incorrect or intermittent electrical signal.
• Corroded or Damaged Wiring/Connectors: Frayed, open, or shorted wiring within the brake pedal switch circuit, or corroded/bent pins in the electrical connector leading to a poor connection.
• Improperly Adjusted Brake Pedal Switch: If the switch is not correctly positioned relative to the brake pedal arm, it may fail to actuate reliably, resulting in an inaccurate signal to the PCM.
• Blown Fuse: A fuse supplying power to the brake light circuit or the brake switch itself may be open, cutting off the necessary voltage.
• PCM/ECM Failure: Although rare, an internal fault within the Powertrain Control Module could prevent it from correctly interpreting the brake switch signal.
• Aftermarket Component Interference: Incorrect installation of aftermarket accessories, such as trailer brake controllers or alarm systems, that tap into the brake light circuit can cause voltage inconsistencies.

How to Diagnose and Troubleshoot

Diagnosis of P1572 requires a systematic approach, combining visual inspection with electrical testing using a digital multimeter (DMM) and an OBD-II scanner:

1. Visual Inspection:
   • Locate the brake pedal switch, which is typically mounted on a bracket near the pivot point of the brake pedal arm under the dash.
   • Carefully inspect the switch body, plunger (if applicable), and its electrical connector for any physical damage, signs of overheating, corrosion, or loose connections.
   • Verify that the switch is properly installed and adjusted, ensuring that the brake pedal arm fully actuates the switch when released and allows it to return to its rest position when pressed.
   • Examine the wiring harness leading to the switch for any chafing, cuts, or signs of rodent damage.
   • Check for any aftermarket wiring splices into the brake light circuit that could introduce resistance or shorts.
   • Locate and inspect all relevant fuses (e.g., “STOP,” “BRAKE,” “IGN”) in both the underhood and cabin fuse boxes for continuity using a DMM.
2. OBD-II Scanner Live Data Analysis:
   • Connect an OBD-II scanner and check for pending, active, or historical Diagnostic Trouble Codes (DTCs). Note any related codes.
   • Access the live data stream and identify the parameter for “Brake Switch Status,” “Stop Lamp Switch,” or similar.
   • With the ignition ON and engine OFF, observe the status while manually pressing and releasing the brake pedal. The status should cleanly toggle between “On/Active” and “Off/Inactive.” An incorrect, stuck, or intermittent reading indicates a fault in the switch or its circuit.
3. Digital Multimeter (DMM) Electrical Testing:
   • Voltage Supply: Disconnect the electrical connector from the brake pedal switch. With the ignition ON, use a DMM to back-probe or test the harness side of the connector for battery voltage (typically 12V) at the power supply pin(s). Refer to the vehicle’s wiring diagram for correct pin identification.
   • Ground Integrity: If applicable, verify a good ground connection at the harness connector’s ground pin by checking for near zero resistance to a known good chassis ground point.
   • Switch Functionality: With the switch disconnected, test the brake pedal switch itself using the DMM’s continuity or resistance setting.
     • For a normally open switch: It should show infinite resistance (open circuit) when the pedal is released and near zero resistance (closed circuit) when the pedal is pressed.
     • For a normally closed switch: It should show near zero resistance (closed circuit) when the pedal is released and infinite resistance (open circuit) when the pedal is pressed.
   • Circuit Integrity (Switch to PCM): If the switch tests good, reconnect it and back-probe the signal wire at the PCM connector (if accessible and safe). Monitor the voltage on the signal wire while pressing and releasing the brake pedal. The voltage should transition cleanly according to the switch type and vehicle specifications. An incorrect reading at the PCM connector, despite a functional switch and proper power/ground at the switch, indicates an open or short in the wiring between the switch and the PCM.

Recommended Repairs and Solutions

Based on the diagnostic findings, the following repairs are typically recommended:

• Replace Brake Pedal Switch: If the brake pedal switch fails DMM testing or shows inconsistent operation in live data, it should be replaced. Ensure the replacement is an OEM-quality part and is correctly installed and adjusted according to the manufacturer’s specifications. Proper adjustment is crucial for reliable operation.
• Repair or Replace Wiring and Connectors: If the diagnostic process identifies corroded, chafed, open, or shorted wiring, or a damaged electrical connector, the affected section of the wiring harness must be repaired or replaced. Use high-quality, automotive-grade connectors and follow best practices for soldering and heat-shrinking if splices are necessary.
• Replace Blown Fuse: If a blown fuse was identified, replace it with a new fuse of the exact specified amperage rating. Investigate the underlying cause of the fuse blowing, as it often indicates a short circuit elsewhere in the brake light system.
• Adjust Brake Pedal Switch: If the switch itself is functional but improperly adjusted, adjust its position to ensure it makes full contact with the brake pedal arm when released and fully retracts when the pedal is pressed. Refer to the vehicle’s service manual for specific adjustment procedures.
• PCM/ECM Replacement: This is a very rare and expensive solution. Only consider PCM/ECM replacement after all other components and wiring have been thoroughly tested and definitively ruled out as the source of the malfunction. If a PCM replacement is necessary, it often requires specialized programming or relearn procedures with a factory-level scan tool.

Mechanic’s Tips: Always consult the vehicle’s specific service manual and wiring diagrams for accurate information regarding brake switch types (normally open/closed), pinouts, voltage specifications, and adjustment procedures. Some vehicles may utilize multiple brake switches for different functions (e.g., one for brake lights, another for PCM input), so it’s essential to identify the specific switch relevant to P1572. After any repair, clear the DTCs from the PCM and perform a thorough test drive to confirm the code does not return and that all affected systems (brake lights, cruise control, shift interlock) are functioning correctly.