Usually safe to drive short term — brakes still work mechanically — but cruise control may be dead, brake lights can misbehave, and the transmission may not unlock the converter clutch smoothly. Repair promptly, especially if brake lights are affected. P0571 is a powertrain code meaning the engine computer sees a problem with the primary brake switch input — the 'A' circuit. In plain terms, the module either never sees the brake signal change as expected, or the wiring/switch feeding that signal has failed, so the computer can no longer trust whether your foot is on the brake.
What P0571 means
The powertrain control module (PCM) monitors a brake pedal position / brake on-off (BPP/BOO) switch so it knows the instant you press the brake. That single input drives several strategies: it drops out of cruise control, releases (unlocks) the torque-converter clutch so the engine doesn't lug or stall, and adjusts idle. Modern systems use two independent brake switch signals for redundancy, and 'Brake Switch A' is the primary of the two. P0571 sets when that A circuit does not behave as the module expects — most commonly when the signal fails to toggle during a functional self-test (on Ford, the key-on engine-running / KOER routine), or when the module detects the circuit is open or shorted. It is a circuit/plausibility fault, not a performance-range fault, so the focus is on the switch itself, its adjustment, and the wiring between the switch and every module that reads it.
Symptoms
- Check-engine light on, sometimes paired with a brake-switch or cruise-related code such as P0572, P0573 or a stored brake-lamp fault
- Cruise control will not engage, or drops out and refuses to reset
- Brake lights stay off, stay on, or behave erratically (the same switch often feeds the stop lamps)
- Shift interlock acts up — you may have to press harder or jiggle the pedal to move the shifter out of Park
- Harsh, early or delayed torque-converter clutch behavior, or a slight lug/shudder when braking because the converter clutch is slow to unlock
- Idle that hangs or dips oddly as you come to a stop
Common causes
- A failed or worn brake pedal position / stop-lamp (BPP/BOO) switch that no longer opens and closes cleanly
- A brake switch that is out of adjustment, so it never fully makes or breaks contact as the pedal moves
- An open in the brake-switch signal circuit — broken wire, corroded terminal, or a backed-out pin at the switch or module connector
- A short to ground (or short to power) on the brake-switch signal wire that pins the input in one state
- A blown stop-lamp / brake-switch fuse or a corroded, loose ground feeding the switch
- A faulty or misconfigured module that shares the brake input (PCM, ABS or body control module), or damage where the circuit is spliced between them
Severity & driving advice
Severity: Moderate — The truck still drives, but loss of a trusted brake signal disables cruise and can upset the torque-converter clutch and shift interlock, so fix it soon.
Can I drive? Usually safe to drive short term — brakes still work mechanically — but cruise control may be dead, brake lights can misbehave, and the transmission may not unlock the converter clutch smoothly. Repair promptly, especially if brake lights are affected.
Diagnostic approach
- Confirm the code and read live brake data — Pull all stored codes and note anything alongside P0571 (a companion brake-switch code such as P0572/P0573 points at the switch or its supply). With the scan tool, watch the brake pedal position / brake on-off (BPP/BOO) PID while a helper presses and releases the pedal — it should cleanly toggle between released and applied. A PID that never changes, or sticks in one state, confirms the fault is live rather than intermittent.
- Check the stop-lamp fuse, feed and the brake lights — Because the same switch usually powers the stop lamps, verify the brake lights work and the stop-lamp / brake-switch fuse is intact. No brake lights plus P0571 strongly suggests a blown fuse, lost switch power, or a bad ground. Repair the supply side before condemning the switch.
- Inspect and test the brake switch itself — Locate the switch on the brake pedal bracket and confirm its adjustment — the plunger must be fully released when the pedal is up and must actuate early in pedal travel. With a meter, verify the switch changes state (continuity/voltage swings as designed) as the pedal is stroked. A switch that is intermittent, sticky, or mis-set is the single most common root cause and is inexpensive to replace.
- Test the signal circuit for opens and shorts — With the switch unplugged, check the brake-switch signal wire back to the module for continuity, for an unintended short to ground, and for a short to power. Wiggle-test the harness and connectors near the pedal box and any inline connectors, since flexing wiring here is a classic intermittent. Confirm the connector terminals are tight and not spread or corroded.
- Verify power and ground at the connector — Confirm the switch has its proper feed and a clean ground with the connector connected (loaded), not just key-on unloaded. Measure voltage drop on the ground side; more than a few tenths of a volt indicates corrosion or a loose ground that can make the input read wrong even when the switch is good.
- Clear, run the self-test, and confirm the repair — After repairing the switch, adjustment or wiring, clear the code and run the module's brake self-test (on Ford, the KOER routine that checks the brake input) or road-test through several stops. Confirm the BPP/BOO PID toggles correctly, cruise control re-engages, brake lights work, and P0571 does not return. If it recurs with a good switch and wiring, suspect a module that shares the brake input.
FAQ
What does 'Brake Switch A' mean in P0571?
Vehicles use two brake switch signals for redundancy so the computer can cross-check that a brake input is real. 'Brake Switch A' is the primary of the two signals. P0571 flags a fault on that primary circuit — either the switch is not toggling, or the wiring feeding the signal is open or shorted. The 'B' circuit shows up in related codes like P0572 (low) and P0573 (high).
Can I keep driving with a P0571 code?
For short trips, usually yes — your brakes work normally because the hydraulic system is separate from this signal. The catch is that cruise control likely won't engage, the brake lights may not work correctly, and the transmission's torque-converter clutch may not unlock smoothly. Because non-working brake lights are both a safety and legal issue, treat this as a soon repair rather than something to ignore.
Is P0571 the same as a bad brake light switch?
Very often, yes. On most vehicles the brake pedal position / stop-lamp switch feeds both the brake lights and the computer's brake input, so a worn, sticking, or mis-adjusted switch is the most common cause of P0571. That said, an open or shorted wire, a blown stop-lamp fuse, or a bad ground can produce the same code with a perfectly good switch, so verify the circuit before replacing parts.
Why does P0571 affect my cruise control and transmission?
The computer uses the brake input to cancel cruise control the moment you touch the pedal and to unlock the torque-converter clutch so the engine won't lug or stall as you slow. When that signal can't be trusted, the module disables cruise as a safety measure and may change how it commands the converter clutch, which you can feel as a slight shudder or a lug when braking.
How is P0571 diagnosed correctly?
Start by watching the brake on-off (BPP/BOO) live data while someone strokes the pedal — it should toggle cleanly. If it doesn't, check the brake switch adjustment and test the switch, then confirm the stop-lamp fuse, power and ground, and check the signal wire for opens or shorts back to the module. Finish by clearing the code and running the module's brake self-test or a road test to confirm the fix.