Comprehensive diagnostic guide for OBD2 code P0571

Overview

• What P0571 typically represents:

  • The fault is associated with the brake pedal/cruise control input circuit. In many OEMs this is described as a brake switch/input circuit (often "A" circuit) malfunction, affecting the brake switch or brake pedal position sensor input used by the PCM/ECU for cruise control or brake input logic.
  • Note: Wikipedia's OBD-II references discuss DTCs in general and identify Powertrain Codes as the group that includes many brake/cruise related inputs, but the specific text of P0571 is not listed in those sections. A related Open Source listing labeled in Portuguese indicates (Brake switch circuit A malfunction), which aligns with the brake switch/ brake pedal input concept. Given OEM variance, P0571 can be described as in some definitions and as in others.
  • Sources: Wikipedia - OBD-II and Powertrain Codes sections; Open Source code listing with brake switch circuit A title.

• Why it matters to the system:

  • The brake pedal input (via brake switch or pedal position sensor) is used to disable cruise when braking and to inform the BCM/PCM of brake activity. A faulty input can cause erratic cruise control behavior or create a fault condition that sets a DTC.

• Cross-check with related codes:

  • P0570 and P0572 (and similar P057x family codes) are commonly discussed in relation to cruise control/brake input circuits. If P0571 appears, inspect the brake input circuit and nearby systems for shared fault sources (wiring, switches, modules). This guide focuses on P0571 but notes related codes may appear concurrently.

Symptoms

• Cruise control behavior:
  • Cruise control will not engage or will disengage unexpectedly.
  • Cruise on/off indicator behavior is inconsistent.
• Brake function indicators:
  • Brake lights may behave abnormally, or there may be intermittent brake-light switch activity indicated by the PCM.
• Other symptoms that can accompany a brake input fault:
  • Intermittent or persistent DTC presence during driving.
  • Possible intermittent ABS/traction system warnings if the vehicle's system shares brake input wiring.
• Emissions/testing impact:
  • Since DTCs can affect the readiness of OBD monitors, the vehicle may fail certain emissions tests if P0571 is present and not cleared or repaired. (Reference: OBD-II Emissions Testing Section)

Probable Causes

• Brake switch A circuit fault (brake light switch or switch wiring/connector issue) - ~40%
  • Most common root cause: misadjusted or failing brake light switch, damaged/contaminated connectors, or corroded terminals in the switch harness.
• Wiring harnesses and connectors to the brake switch or brake pedal position sensor - ~25%
  • Damaged insulation, chafed wires near moving pedal, loose or corroded connections, or poor pin alignment in connectors.
• Brake pedal position sensor (if separate from the switch) or related sensor input to the PCM/BCM - ~15%
  • Sensor failure or intermittent signal, especially in vehicles with non-traditional brake pedal input devices.
• Cruise control module or BCM/PCM input circuit issues - ~15%
  • Ocassionally a software fault, a failed input circuit, or a marginal connection in the module harness.
• Electrical power supply issues (battery/alternator/voltage dips) - ~5%
  • Low or fluctuating voltage can create spurious sensor readings and trigger P0571.

Notes:

• Always review any additional DTCs with P0571, as they can point to a shared root cause (e.g., P0570, P0572, P0598, etc.).

Tools and data to gather before and during diagnosis

• Diagnostic scanner capable of live data for brake switch or pedal position input (Brake Switch, BPS, or equivalent PID; some OEMs list "Brake Pedal Position" or "Cruise Input" data).
• Multimeter (DVOM) to test voltages and resistances on the brake switch circuit and its ground.
• Vehicle service information for brake switch location and circuit wiring (R/H side of pedal assembly; varies by vehicle).
• Basic inspection tools: flashlight, mirror, insulation-safe probing tools, service manual or electrical wiring diagram.
• Optional: oscilloscope for detailed brake switch signal waveform (for intermittent faults).

Step-by-Step Diagnosis

1) Safety and initial data gathering

• Ensure vehicle is in a safe state for inspection (park, parking brake applied, engine off if you're inspecting connectors, but you may need power for certain tests).
• Retrieve the DTCs with the ABS/ESP/cruise control status to see if P0571 appears alone or with other codes (e.g., brake switch related, pedal position, or power/ground faults). Note freeze-frame data and any sensor readings at the time of fault.

2) Visual inspection and basic functional checks

• Inspect brake pedal area and brake light switch for physical damage, misadjustment, or binding.
• Inspect harnesses and connectors to the brake switch and pedal position sensor. Look for damaged insulation, bent pins, corrosion, or loose connectors.
• Check the brake lights function with the pedal depressed; confirm correct operation and brightness. If brake lights stay on or do not illuminate at all, suspect the switch or wiring.
• Inspect related modules and their grounds. A poor ground can produce spurious inputs.

3) Confirm fault with live data

• Using the scan tool, observe the brake input signal ( Brake Switch A / Brake Pedal Position) during pedal action. Note if the PCM/BCM sees a clean, repeatable signal when the pedal is pressed and released.
• If live data is unavailable or inconsistent on your tool, proceed with wiring and component tests to verify continuity and signal integrity.

4) Electrical testing of the brake input circuit

• With ignition on (engine off where safe to do so) and pedal not pressed, test the brake switch circuit for expected voltage and continuity according to the vehicle's wiring diagram.
• Press the brake pedal and verify the signal changes as designed (e.g., a change from one state to another, or a 12V signal to ground, depending on design). Check for intermittent contact or voltage dips.
• Inspect the integrity of the ground path for the brake switch circuit; ensure a solid ground is present and free of corrosion.
• Inspect the wiring harness for chafed wires near moving parts; check for signs of heat damage or moisture intrusion.

5) Inspect related components and potential knock-on effects

• If the brake switch and wiring check out, inspect related systems that share the input circuit (Cruise Control module, PCM/BCM connectors, and their grounds).
• Consider a software/calibration note if the vehicle has field-updated ECUs; verify current software levels per manufacturer service information.

6) Reproduce the fault and verify repair

• After any repair (adjust, clean, replace switch, fix wiring), clear the codes and perform a road test to verify:
  • Cruise control engages and disengages normally.
  • Brake input is read reliably by the PCM/BCM during braking.
  • No reoccurrence of P0571 and no related codes.

Common diagnostic paths and decision points

• If brake lights are functional and the brake switch shows proper operation, yet P0571 persists, suspect wiring between the switch and the PCM/BCM or a failing brake pedal position sensor (if separate) or a module input fault.
• If the brake lights function intermittently or are inoperative, address the brake switch and its wiring first; many P0571 cases resolve with proper switch adjustment or replacement.
• If the brake switch and wiring check out but P0571 remains, you may be dealing with a BCM/PCM input circuit fault or software issue. In this case, consult OEM service information for possible module reprogramming, updates, or recalls.

Repair Options

• Brake switch adjustment or replacement:
  • Align or replace the brake light switch; verify proper travel and actuation with pedal movement.
• Wiring harness repair:
  • Replace damaged wiring, fix chafed insulation, repair or replace affected connectors; ensure proper pin seating and sealing against moisture.
• Brake pedal position sensor replacement (if applicable):
  • Replace sensor and re-torque/attach as per service specs; verify with live data afterward.
• Module-related fixes:
  • Repair or replace the Cruise Control module or BCM/PCM input circuit as per OEM procedure if diagnostics indicate module fault.
  • Software/Calibrations: If OEM confirms, perform required software update or reflash.
• Electrical system housekeeping:
  • Check and address battery health, charging system and any voltage irregularities which could contribute to intermittent sensor readings.

Verification and testing after repair

• Clear P0571 and related codes; perform a road test with varied braking and cruise control operation.
• Confirm live data shows valid brake input during braking and that the Cruise Control module responds correctly to pedal input.
• Ensure no new DTCs appear and that all related systems function normally (brake lights, cruise control, BCM/PCM input signals).
• Confirm emissions readiness if applicable.

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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