How to Tell If Your Brake Light Switch Is Bad

You likely have a bad brake light switch if your brake lights don’t turn on when you press the pedal (or stay on constantly), your cruise control won’t disengage, the shifter is stuck in Park, a push-button start won’t recognize the brake pedal, or you see codes like P0571/P0572/P0573 or P0504. The brake light switch—mounted at the brake pedal or, on some vehicles, at the master cylinder—signals your brake lamps and several safety systems; when it fails or goes out of adjustment, it can create both safety and drivability issues.

What the Brake Light Switch Does

The brake light switch is a small on/off sensor that tells the car when you’re pressing the brake pedal. One internal circuit typically powers the rear brake lamps, while another reports pedal status to the engine/transmission computers for functions like cruise control, transmission shift interlock, push-to-start, ABS/ESC, and brake hold. Because multiple systems depend on it, a faulty or misadjusted switch can show up as lighting, starting, or shifting problems—not just a burned-out bulb lookalike.

Common Symptoms of a Failing Brake Light Switch

The following symptoms are the most frequent clues that the brake light switch or its adjustment/circuit has failed.

• Brake lights don’t turn on when you press the pedal.
• Brake lights stay on constantly, even with the pedal released (can drain the battery overnight).
• Intermittent brake lights: work sometimes, flicker, or require pressing the pedal very hard.
• Can’t shift out of Park unless you use the manual override (shift interlock relies on the switch).
• Cruise control won’t engage or won’t cancel when you tap the brakes.
• Push-button start system won’t let the engine crank even while pressing the brake pedal.
• ABS/ESC/Traction warning lamps illuminate due to implausible brake signal.
• OBD-II/BCM codes such as P0571 (Brake Switch A Circuit), P0572 (Circuit Low), P0573 (Circuit High), or P0504 (Brake Switch A/B Correlation).

One or more of these signs strongly suggests a switch problem or a related wiring/adjustment issue; confirming with a few quick checks will narrow it down.

Quick Checks You Can Do in Minutes

Before replacing parts, these simple checks can help separate a bad switch from blown bulbs, fuses, or misadjustment.

1. Check the bulbs and fuse: Verify all rear brake bulbs and the “STOP”/“BRAKE” fuse. If the high-mount (center) stop light works but the sides don’t, bulbs/sockets may be the issue; if none work, suspect the switch, fuse, or wiring.
2. Look for stuck-on lights: If brake lights are on with the key out, lightly pull the brake pedal toward you; if they turn off, the switch may be misadjusted or the pedal return spring weak.
3. Listen/feel at the pedal: Many switches click. A mushy or no-click feel can indicate misalignment or a broken plunger.
4. Unplug the switch: If the lights go out when you unplug it (and were stuck on), the switch is likely faulty or out of adjustment.
5. Check floor mats/obstructions: An out-of-position mat or trim can prevent full pedal return, keeping the switch “on.”
6. Wiggle test: With someone watching the brake lights, gently move the switch and wiring harness; flickering points to a loose connector or failing switch.

If these checks point to the switch, proceed to testing; if not, consider bulb sockets, grounds, or a wiring fault downstream.

How to Test the Brake Light Switch

A basic electrical test confirms whether the switch has power and changes state when the pedal moves. Many modern switches have two circuits—one normally open (lamps) and one normally closed (control/ECM)—so verify both where applicable.

Tools Needed

Gather the following simple tools to safely diagnose the switch without guesswork.

• Digital multimeter or 12V test light
• Basic hand tools (screwdrivers, small sockets), flashlight
• An assistant to press and hold the brake pedal
• OBD-II scanner (optional, to read/clear codes and view brake switch PID)

With these tools, you can check for power, continuity, and proper switch operation in a few minutes.

Step-by-Step Electrical Test

Use this sequence to isolate the brake light switch as the fault or clear it and move on to the next suspect.

1. Locate the switch: Usually mounted at the top of the brake pedal arm (interior), or less commonly on/near the master cylinder (hydraulic pressure switch).
2. Identify terminals: Consult a wiring diagram if possible. Typically, one terminal has constant 12V (via STOP fuse), and another is the switched output to the brake lamps. A second pair may feed the ECM/cruise.
3. Verify power: With the key off or on (varies by vehicle), check for 12V at the supply pin. No power = check fuses, STOP relay (if equipped), and upstream wiring.
4. Check output: Press the brake pedal. The lamp circuit should switch from 0V to 12V (or continuity should change from open to closed). On the control circuit, the state often flips the other way (closed to open).
5. Measure continuity: Remove the connector and use the ohmmeter across the switch pins. Confirm the normally open lamp circuit closes when pressed; confirm the normally closed control circuit opens when pressed.
6. Wiggle and heat check: Lightly flex the connector or tap the switch; intermittent readings indicate internal wear or a loose terminal fit.
7. Check adjustment: The plunger should be lightly depressed at rest with a small free play. Too tight = lights stay on; too loose = lights delay/never come on. Many switches rotate to self-set; others have a lock ring.
8. If the switch passes: Trace the output wire to the rear, inspect grounds near tail lamp assemblies and trunk/hinge harnesses, and test the bulb sockets for corrosion.

If power is present but the switch won’t change state reliably, replacement is the fix. If power is missing, correct the upstream fuse/relay/wiring before condemning the switch.

Related Fault Codes and What They Mean

Scanning the vehicle can quickly point to the brake switch circuit involved and the likely failure mode.

• P0571: Brake Switch A Circuit malfunction (general fault).
• P0572: Brake Switch A Circuit Low (short to ground/open causing low signal).
• P0573: Brake Switch A Circuit High (short to power/stuck high).
• P0504: Brake Switch A/B Correlation (the two internal circuits disagree—often misadjustment or partial failure).
• Body/ABS codes (manufacturer-specific): May flag “brake pedal signal implausible” or “stop lamp circuit fault” in BCM/ABS modules.

If you see correlation or implausible-signal codes, verify both circuits and the switch adjustment, not just the lamp output.

Replacement and Adjustment Tips

Swapping a brake light switch is usually quick and inexpensive, but correct adjustment is critical for safety and to avoid constant-on lights.

• Disconnect the battery if working near knee airbags or sensitive wiring under the dash.
• Remove the lower dash/kick panel as needed for access; unplug the connector.
• Most pedal-mounted switches twist a quarter turn to release; some use a clip or small bolts.
• Install the new switch and set the plunger: With the pedal at rest, ensure a small preload on the plunger; follow the vehicle’s service procedure for self-adjusting types.
• Verify operation: Lights on with pedal pressed, off when released; confirm shifter release, cruise cancel, and push-to-start recognition.
• Clear codes and test-drive; recheck that the brake lamps aren’t glowing faintly after parking.
• Hydraulic-type switches (older trucks/European models) thread into a port at the master cylinder/line; use thread sealant if specified and bleed brakes only if required by design.

Most switches cost modestly and take 10–30 minutes to replace; the key is ensuring proper alignment and function of both circuits where applicable.

Safety and Legal Implications

A faulty brake light switch isn’t just an annoyance; it can create real hazards and legal exposure.

• No brake lights increase the risk of rear-end collisions.
• Lights stuck on can mask actual braking and drain the battery.
• Shift interlock and cruise control may misbehave, creating drivability risks.
• Many jurisdictions can ticket vehicles for inoperative brake lamps.

Addressing the switch promptly restores critical warning to drivers behind you and ensures safety systems behave as designed.

When It’s Not the Switch

If tests suggest the switch is good, consider other common culprits that mimic switch failure.

• Blown STOP fuse or faulty STOP relay.
• Burned-out bulbs, corroded sockets, or poor grounds at tail lamps.
• Broken wiring in trunk/hatched hinge harnesses or trailer wiring shorts.
• Aftermarket LED bulbs causing CANBUS faults without load resistors (some cars monitor lamp current).
• A separate brake pedal position sensor (used by ABS/ECM) failing while the lamp switch is fine.
• Pedal not returning fully due to misaligned floor mat or weak return spring.
• BCM fault (rare) preventing lamp command despite valid switch signal.

Systematically checking power in, signal out, and lamp circuit continuity helps pinpoint which link in the chain has failed.

Summary

You can suspect a bad brake light switch when brake lamps don’t come on (or stay on), cruise/shift interlock misbehave, push-to-start won’t see the pedal, or codes like P0571–P0573/P0504 appear. Confirm with simple checks—fuses, bulbs, unplug test, and a quick multimeter continuity/voltage test. Replacement is inexpensive and straightforward, but proper adjustment is essential to restore lighting and safety system functions.

How do I tell if my brake switch is bad?

Symptoms of a bad brake light switch include brake lights that stay on constantly, fail to illuminate when the pedal is pressed, or operate intermittently. Other symptoms can include inability to shift out of park in an automatic transmission, cruise control malfunction, an ABS or check engine light on the dashboard, and a vehicle that won’t start with a push-button ignition.
 
Direct Symptoms

• Brake lights don’t turn on: When you press the brake pedal, the brake lights remain off. 
• Brake lights stay on: The brake lights are illuminated even when you’re not pressing the brake pedal. This can cause a parasitic battery drain and premature bulb wear. 
• Inconsistent brake light operation: The brake lights flicker or come on/off sporadically. 

Related System Issues

• Inability to shift out of Park: Many automatic transmissions require a brake pedal signal to release the shift interlock, so a bad switch can prevent shifting. 
• Cruise control malfunction: The cruise control may fail to set or may disengage unexpectedly. 
• Vehicle won’t start: In push-button start vehicles, the brake pedal must be pressed to start the engine. A bad switch can prevent the system from detecting this. 
• Dashboard warning lights: An illuminated ABS light, traction control light, or check engine light can occur, as the brake switch is integrated with these systems. 

What to Do
A faulty brake light switch is a serious safety issue. If you notice these symptoms, it’s important to have the switch inspected and replaced by a qualified mechanic to avoid further electrical problems and ensure road safety.

How do I test if a light switch is bad?

To test a bad light switch, observe for signs like flickering lights, buzzing or crackling sounds, unusual warmth or heat, or a loose switch. If you suspect a problem, turn off the power at the breaker and use a multimeter to test for continuity or voltage. A bad switch will show infinite resistance (no continuity) when turned on or fail to provide correct voltage readings.
 
This video explains how to test a light switch with a multimeter: 1mHowToWith GEOYouTube · Jul 31, 2023
Signs of a Failing Light Switch

• Inconsistent Lighting: Lights that flicker, dim, or only turn on after multiple attempts suggest a faulty switch. 
• Unusual Noises: Buzzing, crackling, or popping sounds from the switch indicate loose connections or internal damage. 
• Warm or Hot to the Touch: A switch that feels warm or hot is a sign of a problem with the wiring or the switch itself. 
• Loose or Damaged: A visibly loose switch or cover, or any signs of damage, also points to a potential issue. 
• Burning Smell or Smoke: A burning smell or smoke coming from the switch is a serious sign of overheating and a potential fire hazard, requiring immediate attention from a professional. 

How to Test a Light Switch (with power off)

1. Turn off the power: to the switch at the circuit breaker. 
2. Remove the switch cover: to access the switch and wires. 
3. Use a multimeter: set to the resistance (ohms) or continuity mode. 
4. Test for continuity: 
   • Place one probe on each screw terminal of the switch. 
   • With the switch in the “off” position, there should be infinite resistance (no connection). 
   • With the switch in the “on” position, there should be zero resistance (a closed circuit), and the multimeter should show a reading or beep. 
5. Interpret the results: If the switch shows infinite resistance (no connection) when it’s in the “on” position, it’s bad and needs replacement. 

This video demonstrates how to troubleshoot a faulty light switch: 56sKletteTechYouTube · May 20, 2024
When to Call an Electrician 

• If you notice signs of a burning smell or smoke.
• If you are uncomfortable or unfamiliar with electrical work.
• If the switch continues to malfunction after replacement.
• For complex wiring issues, especially with 3-way or 4-way switches.

What does the stop light switch tell you?

A stop light (or brake light) switch tells the vehicle’s brake lights to turn on when you press the brake pedal, and it also signals other vehicle systems like cruise control and the shift interlock. It does this by closing an electrical circuit, allowing electricity to flow to the brake lights and other components. A faulty switch can cause brake lights to malfunction, cruise control to fail, or a car with a push-button start to not start.
 
This video demonstrates how to test a brake light switch: 40sCarsNToysYouTube · Sep 14, 2018
What the stop light switch tells the vehicle:

• Brake lights: When you press the brake pedal, the switch completes an electrical circuit, causing the brake lights to illuminate, warning drivers behind you that you are slowing down or stopping. 
• Cruise Control: The switch tells the cruise control system to disengage when you press the brake pedal. 
• Shift interlock: In automatic transmission vehicles, the switch activates a solenoid that allows the gearshift to move out of park when the brake pedal is pressed. 
• Push-Button Start: For cars with a push-button start, the switch sends a signal to the start system to confirm the brake pedal is applied, allowing the car to start. 
• Anti-lock braking system (ABS): The switch provides the ECU (Engine Control Unit) with information about when the brakes are applied, which helps the ABS function correctly. 

Symptoms of a faulty stop light switch:

• Brake lights don’t work: The most common symptom is that the brake lights fail to turn on when the pedal is pressed, according to this YouTube video. 
• Brake lights stay on: The switch can get stuck in the closed position, leaving the brake lights on constantly and draining the battery. 
• Cruise control failure: The cruise control may not work or may not disengage properly. 
• Inability to shift out of park: The shift interlock system will not activate, leaving the car stuck in park. 
• Car won’t start: If you have a push-button start, a faulty switch can prevent the car from starting. 

What happens if the brake light switch goes out?

When a brake light switch fails, it can cause several symptoms, including brake lights not working, brake lights staying on constantly, inability to start a push-button start car, cruise control malfunctions, and issues with shifting out of park in automatic vehicles. These failures create significant safety hazards by preventing other drivers from seeing you slow down and can also drain your battery or lock your gear shift.
 
Symptoms of a Failing Brake Light Switch

• Brake Lights Don’t Illuminate: Opens in new tabThe most common failure is that the brake lights do not turn on when you press the brake pedal. 
• Brake Lights Stay On: Opens in new tabIn some cases, the switch can fail in a way that leaves the brake lights on permanently, even when the car is off, draining the battery. 
• Cruise Control Issues: Opens in new tabThe cruise control system relies on the brake light switch for proper operation, so it may not work or may cancel unexpectedly. 
• Stuck in Park (Automatic Vehicles): Opens in new tabThe brake light switch activates the shift interlock solenoid, which allows you to shift out of Park. If the switch fails, the signal to release the lock is not sent, leaving the vehicle stuck. 
• Car Won’t Start (Push-Button Start): Opens in new tabMany cars with a push-button start require the brake pedal to be pressed to start the engine. A faulty switch will prevent the car from detecting the brake press, resulting in a no-start condition. 
• Dashboard Lights: Opens in new tabSome vehicles may illuminate the ABS or traction control lights, or even the check engine light, due to the faulty brake switch. 

This video demonstrates the symptoms of a bad brake light switch, including the brake lights not working and the shift interlock not working: 59sMechanical BoostYouTube · Jan 10, 2024
Safety Implications

• Reduced Visibility for Other Drivers: Your brake lights are crucial for signaling to drivers behind you when you are slowing down or stopping. 
• Inability to Shift Out of Park: This can leave you stranded in your vehicle. 
• Battery Drain: If the switch fails to keep the brake lights on, it can lead to a parasitic battery drain and a dead battery. 

You can watch this video to see how to diagnose a bad brake light switch and perform a quick test: 51sAuto Repair GuysYouTube · Oct 9, 2019
What to Do

• Inspect Your Brake Lights: Check if they illuminate when you press the brake pedal. 
• Check for Dashboard Lights: Look for warning lights like ABS, traction control, or check engine lights. 
• Try to Start and Shift: Attempt to start your car and see if you can shift out of Park. 
• Consult a Professional: If you notice any of these symptoms, it’s important to have your brake light switch diagnosed and replaced by a qualified mechanic as soon as possible.