P1571

What Does Code P1571 Mean?

Diagnostic Trouble Code (DTC) P1571, often interpreted as “Brake Switch Malfunction,” indicates that the Powertrain Control Module (PCM), also known as the Engine Control Module (ECM) or Vehicle Control Module (VCM) in some applications, has detected an implausible or erratic signal from the brake light switch circuit. The PCM monitors the brake switch input as a critical parameter for various vehicle functions, including but not limited to, torque converter clutch (TCC) operation, cruise control engagement/disengagement, shift interlock release, and providing a brake application signal to other control modules such as the Anti-lock Braking System (ABS) module and Transmission Control Module (TCM).

Specifically, the PCM expects a clear ON/OFF transition from the brake switch signal corresponding to brake pedal actuation. P1571 is typically set when the PCM observes one of the following conditions: the brake switch signal is stuck in an “ON” (brake applied) state when vehicle speed or other sensor inputs suggest the brakes are not applied; the signal is stuck in an “OFF” (brakes released) state when the vehicle is decelerating under conditions that should trigger brake application; or there’s an inconsistent signal that rapidly oscillates between ON and OFF, which is interpreted as an electrical anomaly. This discrepancy can lead to conflicts with other monitored parameters, such as vehicle speed, throttle position, or brake pressure sensor inputs, causing the PCM to identify a circuit malfunction.

Common Symptoms

• Brake Lights Malfunction: Brake lights may stay on constantly, fail to illuminate when the pedal is pressed, or operate intermittently.
• Cruise Control Issues: Inability to set cruise control or cruise control disengaging unexpectedly.
• Transmission Shifting Problems: Difficulty shifting out of “Park,” harsh shifts, or erratic torque converter clutch (TCC) engagement/disengagement, potentially leading to engine stalling at low speeds or stops.
• ABS/Traction Control System (TCS) Warning Lights: Illumination of ABS and/or TCS warning lights, as these systems often rely on accurate brake switch input.
• Reduced Engine Braking or Coasting: The engine management system may misinterpret brake application, affecting fuel cut-off during deceleration.
• Push-Button Start Malfunction: On vehicles with push-button start, the engine may not start without the brake pedal being pressed, and a faulty switch could prevent this.

What Causes the Code P1571?

• Faulty Brake Light Switch: This is the most prevalent cause. The internal contacts of the switch can wear out, become corroded, or the mechanical plunger/actuator can stick, preventing proper signal transmission.
• Wiring Harness Issues: An open circuit, short circuit to ground or voltage, or excessive resistance in the wiring connecting the brake switch to the PCM/BCM. This can be caused by chafing, rodent damage, or improper repairs.
• Poor Electrical Connection: Corroded, loose, or damaged terminals at the brake switch connector or at the PCM/BCM connector.
• Misadjusted Brake Switch: If the brake switch is mechanically misadjusted, it may not actuate correctly (e.g., not fully extending when the pedal is released, or not fully depressing when the pedal is applied), leading to an incorrect signal.
• Blown Fuse: A blown fuse in the brake light circuit or a related power supply circuit can de-energize the switch, preventing it from sending a signal.
• PCM/BCM Malfunction: Although less common, an internal fault within the Powertrain Control Module or Body Control Module (BCM), which processes the brake switch input, could incorrectly interpret the signal.

How to Diagnose and Troubleshoot

Diagnosing P1571 requires a systematic approach, often utilizing a digital multimeter (DMM) and an OBD-II scan tool capable of live data monitoring.

1. Verify the Complaint and Visual Inspection:
   • Begin by confirming the brake light operation. Observe if they function correctly, stay on, or remain off.
   • Locate the brake light switch, typically mounted on a bracket near the top of the brake pedal arm.
   • Visually inspect the switch for physical damage, cracks, or signs of wear on its plunger. Check for any obstructions preventing full pedal travel or switch actuation.
   • Inspect the wiring harness leading to the switch for signs of chafing, cuts, or corrosion. Verify the connector is securely seated.
2. Retrieve and Analyze Diagnostic Trouble Codes (DTCs):
   • Connect an OBD-II scan tool and confirm P1571 is present. Check for any other related codes in the ABS, BCM, or TCM modules that might offer additional diagnostic direction.
   • Clear the codes and road test the vehicle under conditions where the code typically sets, if possible, to verify if the code returns.
   • Utilize the scan tool’s live data function to monitor the “Brake Switch Status” or “Brake Pedal Position” parameter. Observe its state as the brake pedal is pressed and released. It should reliably toggle between “ON” (or “Applied”) and “OFF” (or “Released”). If it shows “ON” constantly when the pedal is up, “OFF” constantly when the pedal is down, or erratic switching, this confirms an issue within the circuit.
3. Electrical Circuit Testing with DMM:
   • Power Supply Test: With the ignition in the RUN position (engine off), disconnect the brake switch electrical connector. Using a DMM, check for battery voltage (B+) at the power input wire(s) to the switch. Consult the vehicle’s wiring diagram for correct pin identification. If no voltage is present, trace the circuit back to the fuse box and check relevant fuses.
   • Ground Circuit Test: If applicable, test for good ground continuity at the ground pin of the connector using the DMM’s resistance (ohms) function.
   • Switch Continuity/Resistance Test: Disconnect the brake switch and remove it from its mounting. Using the DMM, test for continuity across the switch terminals. For most normally open (NO) switches, there should be infinite resistance (open circuit) when the plunger is extended (pedal released) and near-zero resistance (closed circuit) when the plunger is fully depressed (pedal applied). For normally closed (NC) switches, the opposite is true. For multi-pin switches with multiple contacts (e.g., one for brake lights, one for PCM), test each set of contacts according to the wiring diagram.
   • Signal Circuit Integrity: If the switch tests good off the vehicle, back-probe the signal wire at the switch connector with the switch reconnected. Monitor the voltage on this wire while pressing and releasing the brake pedal. The voltage should reliably transition between expected states (e.g., 0V and B+, or vice-versa, depending on the switch design).
4. Brake Switch Adjustment Verification:
   • Even if the switch tests electrically sound, incorrect mechanical adjustment can cause P1571. Ensure the switch plunger is fully extended when the brake pedal is at its resting position and fully depressed when the pedal is pressed. Many modern brake switches are a push-in design that self-adjusts when installed correctly, ensuring the plunger is compressed to the correct initial position.

Recommended Repairs and Solutions

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

1. Replace the Brake Light Switch: This is the most frequent repair for P1571. Ensure the replacement part is a high-quality OEM or reputable aftermarket equivalent.
   • Mechanics’ Tip: When installing a new brake switch, particularly the self-adjusting push-in types, ensure the brake pedal is in its fully released position. Install the switch by pushing it firmly into its mounting hole until it clicks into place. This allows the switch plunger to find its correct initial position against the brake pedal arm. For threaded switches, adjust it so that the brake lights illuminate with minimal pedal travel and extinguish completely when the pedal is fully released, ensuring no residual pressure on the plunger.
2. Repair or Replace Damaged Wiring/Connectors: If the DMM tests reveal an open, short, or high resistance in the wiring harness, repair the affected section using proper automotive wiring repair techniques (e.g., soldering and heat-shrinking). Replace damaged connectors if terminals are corroded or loose. Always use appropriate gauge wire and connectors.
3. Check and Replace Fuses: If the brake switch is not receiving power, identify and replace any blown fuses in the relevant brake light or control module power circuits. Always replace fuses with one of the correct amperage rating.
4. Address Misadjustment: If the existing brake switch is functional but misadjusted, carefully adjust it according to manufacturer specifications. If it’s a self-adjusting type and found to be misadjusted, it may indicate a worn switch or a problem with the brake pedal assembly itself.
5. PCM/BCM Replacement (Last Resort): If all external components (switch, wiring, connectors, fuses) test unequivocally good, and the P1571 code persists with confirmed symptoms, then a PCM or BCM malfunction may be suspected. This is a rare occurrence for P1571 and should only be considered after exhaustive testing has ruled out all other possibilities. Module replacement often requires programming by a dealership or specialized technician.