P0571: brake switch A circuit and the cruise control that quit

The brake pedal switch is one of the simplest components on a car and one of the most consequential when it fails, because so many systems ask it the same question: is the driver braking right now?

Related: cruise control, when it helps.

Who asks, and what happens when the answer is wrong

• Cruise control disengages on brake application. A switch reading “braking” constantly means cruise control will not engage at all.
• The gear selector interlock requires the brake to be pressed before leaving Park. A switch that never reads “braking” means the lever will not move.
• The stop lamps, on most vehicles, though many use a second switch or a redundant contact.
• The engine management system, which uses brake status for idle strategy and for torque management.
• Start-stop systems, which use it to decide when to restart: auto start-stop diagnostics.
• Adaptive cruise and emergency braking on vehicles with driver assistance: speed limiter and adaptive cruise.

That list explains the classic presentation: “the cruise control stopped working and the car will not come out of Park.” Two complaints, one switch.

Two contacts, one switch

Most brake switches contain two independent circuits that operate in opposite senses — one closes when the pedal is pressed, the other opens.

The redundancy exists so the ECU can cross-check. If both contacts report the same state, or the transition between them does not make sense, a fault is logged.

That is why P0571 often appears alongside P0572, P0573 or a manufacturer-specific correlation code: they are reporting the two halves disagreeing.

Adjustment is the most common cause

The switch is adjustable, and it drifts.

It mounts on a bracket behind the pedal with a threaded body or a snap-in plunger. Over time the bracket flexes slightly, the pedal stop pad compresses, and the switch no longer operates at the right point in the pedal travel.

Symptoms of a mis-adjusted switch:

• Stop lamps on constantly, or lit with the pedal untouched.
• Stop lamps that need a firm press before they come on.
• Cruise control that will not set.
• The car that will not shift out of Park.

Adjustment is usually free and takes minutes on most vehicles. Check it before buying anything.

The rubber pad that disappears

A specific and common failure worth knowing.

Many vehicles have a small rubber or plastic pad on the pedal arm that the switch plunger rests against. That pad degrades and eventually falls out — often found on the driver’s footwell carpet.

Without the pad, the plunger extends too far, and the switch behaves as though the brake is permanently pressed. The stop lamps stay on, the battery goes flat overnight, and the cruise control refuses to work.

If your stop lamps are on with the car parked and locked, look for a missing pedal pad before anything else. It is a very cheap part, and it explains a mysterious battery drain: parasitic battery drain diagnostics.

Other causes

The connector, in the footwell where feet and cleaning water reach it.

The switch itself, whose contacts wear out after tens of thousands of operations.

Wiring, particularly if anything has been installed under the dashboard — alarms and remote starters commonly tap the brake circuit.

What to check in the data

Brake switch status is exposed on many vehicles as a simple on/off value.

Test	Expected
Pedal untouched	Off
Pedal pressed lightly	On
Pedal released	Back to off immediately
Both contacts, where visible	Always opposite states

Watch it while pressing the pedal slowly. A switch that changes state only at the very bottom of the travel, or one that flickers, is mis-adjusted or worn.

The knock-on codes

Because so many systems use brake status, a failed switch produces codes far from the pedal — transmission codes about range selection, engine codes about torque management, and cruise control faults.

P0703 and P0706 brake switch and range sensor covers the transmission side specifically.

If you have a scattering of codes across systems and a brake switch code among them, fix the switch first and re-scan.

Where to start

1. Look at the stop lamps with the pedal untouched.
2. Check for the rubber pedal pad — a missing one explains a lot.
3. Check the switch adjustment.
4. Watch brake status in live data through a slow pedal press.
5. Inspect the connector.
6. Replace and re-adjust if needed, then clear the downstream codes.

Pair an ELM327 adapter, open the free tier of the app and watch brake switch status while you press the pedal. If the state does not change cleanly, you have found a fault that was making three unrelated systems look broken.

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