Brake Switch "A" Circuit
This code indicates a malfunction in the primary brake switch circuit, which tells the engine control module (ECM) when the brake pedal is depressed. The ECM sets this code when it detects an improper voltage signal or a lack of expected signal change from the brake switch 'A' circuit during vehicle operation.
This code is considered high priority because it can cause the brake lights to fail, creating a significant rear-end collision hazard. Additionally, it may prevent the vehicle from shifting out of park or disable the push-button start system.
Common symptoms
- Check Engine Light is illuminated
- Brake lights remain on continuously or do not illuminate at all
- Cruise control system is disabled or fails to engage
- Inability to shift the transmission out of the 'Park' position
- Push-button start systems may fail to crank the engine
Likely causes
- Defective or misadjusted brake light switch
- Blown fuse for the brake light circuit
- Damaged, corroded, or loose wiring and connectors in the brake switch circuit
- Burned out brake light bulbs causing abnormal circuit resistance
- Faulty engine control module (ECM) or body control module (BCM)
What to check before replacing parts
Inspect the brake light circuit fuses.
Look for: A blown fuse, which indicates a short circuit or overload in the brake light system.
Check the brake light bulbs.
Look for: Burned out filaments or incorrect bulb types that could alter the circuit's expected electrical resistance.
Visually inspect the brake switch and its connector under the dashboard.
Look for: Loose connections, damaged wires, or a switch that has backed out of its mounting bracket and lost physical contact with the brake pedal.
Test the brake switch for continuity using a digital multimeter.
Look for: A lack of continuity change when the switch plunger is manually pressed and released, indicating an internal switch failure.
Measure the voltage at the brake switch connector.
Look for: Absence of reference voltage or battery voltage on the supply side, pointing to a wiring issue between the fuse panel and the switch.
This page explains the generic P0571 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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