P0571

What Does Code P0571 Mean?

The diagnostic trouble code P0571 indicates a “Cruise Control/Brake Switch A Circuit Malfunction.” This code is set by the Powertrain Control Module (PCM) or Engine Control Module (ECM) when it detects an electrical anomaly within the primary circuit of the brake pedal position (BPP) switch, also commonly referred to as the stop light switch. The brake switch serves as a critical input to various vehicle control systems, signaling when the brake pedal is depressed. Circuit ‘A’ typically refers to the main signal path providing brake pedal status to the PCM for purposes beyond just illuminating the brake lights, such as disengaging cruise control, controlling torque converter clutch (TCC) operation, and assisting in gear shift interlock systems. The PCM monitors the voltage or resistance across this circuit. If the PCM detects a signal that is inconsistent with expected parameters (e.g., an open circuit, a short circuit, an out-of-range voltage, or a signal that fails to transition properly when the brake pedal is activated/deactivated), it interprets this as a malfunction and illuminates the Malfunction Indicator Lamp (MIL), storing the P0571 code.

Common Symptoms

• Cruise control system failing to engage.
• Cruise control system failing to disengage when the brake pedal is pressed.
• Malfunction Indicator Lamp (MIL) illumination.
• Difficulty shifting out of ‘Park’ (in automatic transmissions, if the brake switch is integrated with the shift interlock).
• Erratic or incorrect operation of the torque converter clutch (TCC), potentially leading to harsh shifts or reduced fuel economy.
• Brake lights may or may not illuminate (depending on whether the affected circuit is shared with the brake lights or if it’s a separate signal circuit for the PCM).
• In some rare cases, unintended acceleration if the PCM erroneously believes the brakes are not applied.

What Causes the Code P0571?

• Faulty Brake Pedal Position (BPP) Switch: The most common cause is an internal electrical failure, wear, or mechanical degradation of the brake switch itself, leading to intermittent or complete loss of signal.
• Wiring Harness Issues: An open circuit, short to ground, short to voltage, or high resistance in the wiring harness connecting the brake switch to the PCM. This can be due to chafing, corrosion, or physical damage.
• Corroded or Loose Electrical Connectors: Poor electrical contact at the brake switch connector or the PCM connector pins due to corrosion, improper seating, or damage.
• Incorrect Brake Switch Adjustment: If the brake switch is adjustable, an improper mechanical adjustment can prevent the switch from fully activating or deactivating as the pedal is pressed and released.
• Blown Fuse: A blown fuse supplying power to the brake switch circuit can cause a lack of signal or an open circuit condition.
• Powertrain Control Module (PCM) Failure: While less common, an internal fault within the PCM preventing it from correctly interpreting the brake switch signal or supplying the correct reference voltage can also trigger this code.

How to Diagnose and Troubleshoot

Diagnosis of P0571 requires a methodical approach, utilizing an OBD-II scanner and a digital multimeter (DMM).

1. Retrieve and Document DTCs: Connect an OBD-II scanner to read all stored codes and freeze frame data. Note any other related codes, as they might provide additional clues. Clear the codes to see if P0571 immediately returns.
2. Visual Inspection: Begin by visually inspecting the brake pedal position switch, its mounting, and the associated wiring harness. Check for physical damage to the switch, signs of corrosion at the connector, or chafing/fraying of the wires. Ensure the switch plunger or arm is properly engaged with the brake pedal arm and that there is adequate travel. Inspect relevant fuses in the under-hood and cabin fuse boxes for continuity.
3. Scanner Live Data Analysis: With the ignition ON and engine OFF (KOEO), access the scanner’s live data stream. Monitor the “Brake Pedal Position” or “Stop Light Switch” parameter. Operate the brake pedal through its full range of motion while observing the parameter. It should reliably toggle between “ON/OFF” or “Applied/Released” states. An inconsistent, stuck, or non-responsive reading indicates a fault in the switch or its circuit.
4. Digital Multimeter (DMM) Testing at the Switch:
   • Power Supply Check: Disconnect the brake switch connector. Using a DMM, check for proper reference voltage (typically 12V or 5V, depending on the system design) at the power supply wire within the vehicle’s harness connector. Also, check for a good ground connection at the appropriate pin.
   • Switch Functionality Test (Disconnected): With the switch still disconnected, use the DMM in continuity or resistance mode. Test across the relevant terminals of the brake switch itself. As the brake pedal is manually pressed to simulate actuation, the switch should reliably open and close, showing a change from open circuit to continuity (or vice-versa, depending on whether it’s a normally open or normally closed switch).
   • Signal Voltage Check (Connected): Reconnect the brake switch. Backprobe the signal wire at the brake switch connector (or at the PCM connector, if accessible). With the ignition ON, measure the voltage on the signal wire while the brake pedal is released and then pressed. The voltage should reliably transition between two distinct states (e.g., 0V and 12V, or 0V and 5V, or vice-versa).
5. Wiring Integrity Check: If the brake switch tests good, suspect the wiring. With the battery disconnected and the PCM connector unplugged, use the DMM to check for continuity between the brake switch connector and the corresponding pin at the PCM connector for the signal wire. Simultaneously, check for shorts to ground or shorts to power on the signal wire.

Recommended Repairs and Solutions

• Replace the Brake Pedal Position (BPP) Switch: Based on diagnostic findings, replacing a faulty brake switch is the most common and often effective solution. Ensure the replacement switch is of OEM quality or a reputable aftermarket equivalent. Follow manufacturer specifications for installation, especially concerning any adjustment procedures to ensure correct plunger/arm engagement and proper switch operation.
• Repair or Replace Damaged Wiring: If the visual inspection or DMM testing reveals damaged, corroded, or shorted wiring, the affected section of the harness should be professionally repaired or replaced. Use appropriate automotive-grade wire, connectors, and splicing techniques (e.g., soldered connections with heat shrink tubing).
• Clean or Replace Connectors: If corrosion or poor terminal tension is found in the connectors, attempt to clean them thoroughly using electrical contact cleaner and a small brush. If damage is extensive, replace the connector.
• Adjust Brake Switch: If the original brake switch is functional but improperly adjusted, consult the vehicle service manual for the correct adjustment procedure. This usually involves ensuring the switch actuates at the specified brake pedal travel.
• Replace Blown Fuse: If a blown fuse was identified, replace it with a fuse of the correct amperage. Investigate the cause of the blown fuse if it’s not immediately apparent, as a short in the circuit could be the underlying issue.
• PCM Repair or Replacement: Only consider PCM replacement after exhausting all other diagnostic possibilities and confirming an internal PCM fault through manufacturer-specific diagnostic procedures. This is a rare cause for P0571.

After any repair, clear the DTCs from the PCM using an OBD-II scanner. Perform a test drive, ensuring the cruise control engages and disengages properly, and monitor live data to confirm the brake switch signal is now accurate and consistent. Recheck for any pending or stored codes.