P0571 Cruise Control / Brake Switch "A" Circuit Malfunction

The P0571 diagnostic trouble code indicates that the engine or powertrain control module has detected a malfunction in the Cruise Control / Brake Switch “A” circuit.

The brake switch is one of the most important inputs used by the cruise-control system. The control module needs to know when the brake pedal is pressed so it can immediately cancel cruise control.

When P0571 is stored, the module has detected that the signal from the Brake Switch “A” circuit does not behave as expected.

This is a broader code than P0572 and P0573. Those codes identify a specifically low or high electrical condition, while P0571 indicates a general malfunction.

What Is the Brake Switch "A" Circuit?

The brake pedal switch is normally mounted near the top of the brake pedal.

Although it is often called simply the “brake switch,” many vehicles use a switch assembly with multiple electrical contacts or circuits.

One circuit may operate the brake lamps while another circuit sends information to the ECU or other control modules.

This means the brake lights can sometimes work correctly even when the ECU is detecting a fault in the circuit associated with cruise control.

The control module monitors the brake-switch signal and expects it to change state at the appropriate time.

When the brake pedal is released, the circuit should indicate one condition.

When the brake pedal is pressed, it should indicate another.

If the expected transition does not occur, P0571 may be set.

Why Does Cruise Control Need the Brake Switch?

Cruise control must be able to disengage when the driver applies the brakes.

The brake-switch signal therefore acts as a safety input.

Under normal operation:

Brake pedal released → cruise control can remain active

Brake pedal pressed → cruise control is cancelled

If the ECU receives an implausible brake-switch signal, it may disable cruise control altogether.

This is intentional. The system should not continue controlling vehicle speed if it cannot reliably determine whether the driver is braking.

What Does “Circuit Malfunction” Mean?

P0571 does not tell you that the circuit is simply high or low.

Instead, the control module has detected a condition that does not match the expected operation of the brake-switch circuit.

Possible examples include:

• Brake signal does not change when the pedal is pressed

• Signal changes at the wrong time

• Two brake-switch signals disagree

• Signal is intermittent

• Circuit is open

• Circuit is shorted

• Switch contacts are internally faulty

• Electrical values are outside the expected range

The exact diagnostic logic depends on the vehicle.

Common Causes of P0571

Faulty brake pedal switch

The internal contacts can wear out or become electrically unreliable.

Incorrect switch adjustment

A switch that is installed or adjusted incorrectly may change state too early or too late.

Damaged wiring

A broken, chafed, pinched, or partially broken wire can interrupt the brake signal.

Loose or corroded connector

Poor terminal contact can cause an intermittent or invalid signal.

Short to ground or power

The signal may be forced into an electrical state that the ECU does not expect.

Incorrect replacement switch

Some brake switches have different internal contact arrangements despite looking physically similar. Installing an incorrect switch can produce a persistent P0571.

Brake pedal mechanical problem

If the pedal does not return completely or moves abnormally, the switch may not operate correctly.

Fuse or power-supply problem

Depending on the vehicle design, the brake-switch circuit may share or depend on a specific power supply or fuse.

Communication problem

Modern vehicles may distribute brake-switch information between the BCM, ABS module, ECU, and other controllers.

ECU/PCM fault

An internal module fault is possible, but it should normally be investigated only after the external circuit has been tested.

Symptoms of P0571

The most obvious symptom is often inoperative cruise control.

Possible symptoms include:

• Cruise control cannot be activated

• Cruise control suddenly stops working

• SET function does not work

• Cruise control disengages unexpectedly

• Check Engine Light

• Cruise-control warning light

• Brake lights remain on

• Brake lights do not operate

• Cruise control becomes intermittent

• Other modules report an implausible brake-switch signal

The engine may otherwise run completely normally.

This is because P0571 primarily concerns the brake/cruise-control input circuit, rather than fuel injection, ignition, or engine performance.

Can the Brake Lights Still Work?

Yes.

This is an important diagnostic point.

A vehicle may use separate contacts within the brake switch.

For example, one contact can operate the brake lamps while another provides a signal to the ECU.

Therefore, seeing the brake lights working does not prove that the entire brake-switch circuit is functioning correctly.

The specific “A” circuit must be tested.

How to Diagnose P0571

The best starting point is to scan all vehicle modules, not just the engine ECU.

Look for additional codes from:

• ABS

• BCM

• TCM

• Steering-wheel module

• Cruise-control system

• Communication network

If another module reports a brake-switch or communication problem, it may explain why P0571 was generated.

Next, check the brake-switch status using scan-tool live data if available.

With the pedal released, the ECU should show the expected released state.

Press the pedal.

The status should change immediately and consistently.

If the data remains unchanged, changes with a delay, or changes incorrectly, the brake-switch circuit requires further investigation.

Check the Brake Switch Mechanically

Before performing complicated electrical tests, inspect the switch installation.

Check:

• Brake pedal return

• Switch mounting

• Switch adjustment

• Switch plunger position

• Pedal movement

• Physical damage

• Loose mounting

A switch that is positioned incorrectly can behave electrically like a defective switch.

Some brake switches have a self-adjusting mechanism, so following the manufacturer's installation procedure is important.

Test the Switch Electrically

The switch should be tested using the vehicle's wiring diagram.

Depending on the design, the technician may need to check:

• Supply voltage

• Output voltage

• Continuity

• Contact state with the pedal released

• Contact state with the pedal pressed

If the switch contains multiple contacts, each circuit must be tested separately.

Do not assume that testing the brake-lamp circuit automatically tests the cruise-control circuit.

A switch can have one functioning contact and another defective contact.

Check for Signal Transition

One of the most useful tests for P0571 is determining whether the electrical state changes when the brake pedal moves.

For example, the circuit might normally behave like:

Pedal released → signal state A

Pedal pressed → signal state B

If it remains at state A regardless of pedal position, the switch or circuit may be open or otherwise faulty.

If it remains at state B, the switch may be stuck electrically, incorrectly adjusted, or the signal may be shorted.

If the signal changes but does so inconsistently, investigate the switch, connector, and wiring for an intermittent fault.

Inspect the Wiring Around the Pedal

The harness near the brake pedal should be inspected carefully.

Look for:

• Chafed insulation

• Broken wires

• Stretched wires

• Pinched harness sections

• Loose terminals

• Previous repairs

• Aftermarket electrical connections

Because the wiring is located near a moving pedal mechanism, physical interference can sometimes damage the harness.

Check the Connector

Disconnect the brake-switch connector and inspect the terminals.

Look for:

• Corrosion

• Water contamination

• Bent pins

• Loose terminals

• Terminals pushed backward

• Damaged connector locks

A connector can look clean but still have insufficient terminal tension.

If the fault is intermittent, gently moving the connector or harness while monitoring the brake-switch signal can help identify the problem, provided the manufacturer's diagnostic procedure permits this.

Check the ECU Input

If the brake switch and wiring test correctly, the next question is whether the correct signal actually reaches the control module.

This is where a wiring diagram becomes essential.

The technician can verify continuity and signal integrity between the brake-switch circuit and the appropriate module terminal.

If the switch generates the correct signal and the wiring delivers that signal correctly, but the ECU continues to report a malfunction, the module or its communication with another controller may need further investigation.

P0571 vs. P0572

These codes are closely related.

P0571 – Cruise Control / Brake Switch “A” Circuit Malfunction

A general malfunction has been detected in the circuit.

P0572 – Cruise Control / Brake Switch “A” Circuit Low

The module specifically detects an electrical signal that is lower than expected.

P0571 is therefore broader.

If the diagnostic data clearly shows that the circuit is being pulled low, P0572 may be more appropriate depending on the vehicle's diagnostic strategy.

P0571 vs. P0573

P0573 – Cruise Control / Brake Switch “A” Circuit High

The module detects a signal higher than the expected range.

P0571 does not necessarily mean that the signal is high or low. It indicates that the overall behavior of the circuit is incorrect.

This distinction is useful because P0571 can result from incorrect switching, an intermittent connection, an open circuit, a short, or disagreement between multiple brake-switch signals.

P0571 and Dual Brake-Switch Signals

Some modern vehicles use multiple brake-switch signals specifically to improve reliability.

The ECU can compare the signals.

For example, one circuit may indicate that the brake pedal is released while another indicates that it is pressed.

If the two signals disagree for longer than the permitted time, the ECU can identify the condition as implausible and store a fault.

In such a system, P0571 may therefore be caused by one faulty contact even though another brake-switch circuit continues to work normally.

This is another reason why checking only the brake lamps is not sufficient.

Can a Bad Brake Switch Cause P0571?

Yes.

The brake switch is one of the first components that should be tested.

However, replacing it without checking the circuit can result in an unnecessary repair.

If the switch works correctly but the signal is lost between the switch and the ECU, the actual problem is in the wiring or connector.

Likewise, if the wiring and switch are correct but the modules disagree about the brake status, the fault may involve communication or another control module.

Can P0571 Be Caused by a Wiring Problem?

Yes.

A wiring problem is a common possibility.

The circuit may be:

• Open

• Shorted to ground

• Shorted to power

• Intermittently connected

• High resistance

• Damaged near a connector

A partially broken wire is particularly difficult to diagnose because it may pass a basic continuity test while failing when the harness moves.

Is P0571 Serious?

P0571 is generally not an indication of internal engine damage, but it should not be ignored.

If the fault only disables cruise control and the brake lights work correctly, the immediate consequence may be limited to loss of cruise-control functionality.

However, if the brake-switch circuit is also affecting the brake lights, the issue becomes a safety-critical fault.

Before continuing to drive normally, verify that the brake lamps illuminate correctly and consistently whenever the brake pedal is pressed.

What Is the Correct Repair?

The correct repair depends on the actual test result.

Possible repairs include:

• Replacing the brake pedal switch

• Correctly adjusting the switch

• Repairing damaged wiring

• Repairing or replacing connector terminals

• Correcting a power or ground problem

• Replacing an incorrect switch with the correct specification

• Repairing a communication fault

• Repairing a related control module

• Repairing or replacing the ECU/PCM in rare cases

The code itself does not identify which component must be replaced.

Final Diagnostic Summary

P0571 means the control module has detected a malfunction in the Cruise Control / Brake Switch “A” circuit.

The brake switch is an obvious suspect, but the diagnosis should include the switch adjustment, multiple switch contacts, wiring, connectors, power supply, and communication between control modules.

A practical diagnostic sequence is:

Check brake-light operation → scan all modules → monitor brake-switch live data → inspect switch adjustment → test the switch contacts → verify signal voltage/state → inspect wiring and connectors → verify the signal at the control module.

If the switch does not change state correctly, the switch or its adjustment is the likely problem. If the switch works correctly but the signal does not reach the ECU correctly, investigate the wiring and connectors. If the physical circuit is proven good but the control modules still disagree, then module communication or an ECU/BCM/ABS-related issue becomes more relevant.

The key point with P0571 is that it is a general brake/cruise-control input malfunction code. It should be diagnosed by following the actual signal through the circuit rather than assuming that the brake switch itself has failed.