P0571 — Brake Switch 'A' Circuit

Quick answer

• What the code means: The computer detects an issue with the primary brake switch circuit, indicating it's not sending the expected signal when the brake pedal is pressed or released.
• Severity: Get it diagnosed promptly. While you might still have brake lights, critical safety systems like cruise control and ABS can be affected.
• Most common cause: A faulty brake light switch.

What the P0571 code means

The P0571 diagnostic trouble code (DTC) officially stands for "Brake Switch 'A' Circuit Malfunction". In simpler terms, your vehicle's powertrain control module (PCM) or engine control unit (ECU) has detected a problem with the electrical circuit of the primary brake pedal switch. This switch is crucial because it tells the vehicle's various systems when the brake pedal is being pressed or released. If the signal from this switch is incorrect, inconsistent, or absent, it can affect several functions, not just the brake lights.

Modern vehicles rely heavily on the brake switch signal for a wide array of operations. It communicates with the engine computer, transmission control module, anti-lock braking system (ABS), traction control system (TCS), cruise control system, and even the push-button start system in some cases. When the PCM sees an irrational signal or no signal at all from the 'A' circuit of the brake switch, it triggers the P0571 code.

Common symptoms

• Brake lights not working, illuminated constantly, or intermittent.
• Cruise control not engaging or disengaging.
• Inability to shift out of Park (on automatic transmissions).
• Engine may not start (if equipped with push-button start and requiring brake pedal input).
• Malfunction Indicator Lamp (MIL) or "Check Engine Light" illuminated.
• ABS or Traction Control warning lights illuminated (less common, but possible).

Common causes

1. Faulty Brake Light Switch: This is by far the most common culprit. The internal contacts can wear out or become corroded, leading to intermittent or complete failure.
2. Wiring Harness Issues: Damaged, frayed, shorted, or open circuits in the wiring leading to or from the brake switch. Rodent damage, chafing, or improper installation can cause these problems.
3. Loose or Corroded Connector: The electrical connector at the brake switch can become loose, dirty, or corroded, preventing a proper electrical connection.
4. Improperly Adjusted Brake Switch: The brake switch might be physically out of adjustment, meaning it's not being fully activated or deactivated by the brake pedal.
5. Faulty PCM/ECU: While rare, an internal fault within the vehicle's main computer could incorrectly interpret the brake switch signal or fail to process it. This is usually a last resort diagnosis after ruling out all other possibilities.

How a technician diagnoses it

1. Verify the Complaint and Perform a Visual Inspection: The technician will first confirm the symptoms and visually inspect the brake switch, its mounting, and the surrounding wiring for any obvious damage, disconnections, or improper adjustment.
2. Scan for Codes and Freeze Frame Data: A professional scan tool will be connected to retrieve the P0571 code and any other related DTCs. Freeze frame data will be reviewed to see the operating conditions (engine speed, vehicle speed, etc.) when the code was set, which can provide valuable clues.
3. Live Data Stream Analysis: The technician will monitor the brake switch PID (Parameter ID) in the scan tool's live data stream. When the brake pedal is pressed, the switch status should change from "OFF" to "ON" (or similar terminology). If it doesn't change, or changes erratically, it points to an issue with the switch or its circuit.
   • Good Value: Brake switch status changes reliably and instantaneously with pedal depression/release.
   • Bad Value: Brake switch status remains "OFF" or "ON" regardless of pedal movement, or changes intermittently.
4. Circuit Testing with a Digital Multimeter (DMM):
   • Power Supply Check: The technician will check for proper voltage supply (typically battery voltage, ~12V) at the brake switch connector.
   • Ground Check: Ensure a good ground connection at the switch.
   • Continuity Check: With the switch disconnected, the technician will check for continuity through the switch itself as the pedal is pressed and released. They will also check for continuity in the wiring from the switch to the PCM/other relevant modules.
   • Good Value: Expected voltage present, good ground, continuity changes appropriately with switch activation.
   • Bad Value: No voltage, poor ground, open circuit in wiring, or no continuity change through the switch.
5. Brake Switch Adjustment Check: If the switch is adjustable, the technician will ensure it's correctly positioned to engage and disengage at the appropriate point of pedal travel.

Typical repair cost

The repair cost for a P0571 code can vary depending on the specific cause and your vehicle's make and model:

• Brake Light Switch Replacement:
  • Parts: $15 - $75
  • Labor: $40 - $120
  • Total: $55 - $195
• Wiring Repair:
  • Parts: $5 - $30 (for connectors, wire, tape)
  • Labor: $80 - $300 (depending on complexity and diagnostic time)
  • Total: $85 - $330
• PCM/ECU Replacement (rare): This is significantly more expensive, potentially ranging from $800 to $2,000+ for parts and labor, plus programming.

These estimates do not include diagnostic fees, which typically range from $80 to $150.

Can I keep driving with this code?

It is not advisable to keep driving with a P0571 code without immediate diagnosis and repair. While your brakes might still function mechanically, the code indicates a problem with the brake switch signal, which is critical for several safety systems:

• No brake lights/constant brake lights: This is a major safety hazard, as other drivers won't know when you are slowing or stopping.
• Cruise control malfunction: This can be inconvenient and potentially dangerous if it fails to disengage.
• Shifting issues: Being unable to shift out of Park can leave you stranded.
• No-start condition: If your car requires brake pedal input to start, you won't be able to drive at all.
• Reduced ABS/TCS effectiveness: While less common, an incorrect brake switch signal could confuse these systems, although the primary brake function usually remains intact.

Given the potential impact on safety and vehicle operation, get this code diagnosed and repaired as soon as possible.

Related codes

• P0572: Brake Switch 'A' Circuit Low
• P0573: Brake Switch 'A' Circuit High
• P0504: Brake Switch 'A' / 'B' Correlation
• P0703: Torque Converter Clutch Brake Switch 'B' Circuit Malfunction
• C0277: Brake Lamp Switch Circuit

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