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Summary
  • Diagnostic trouble code (DTC) P0504 stands for “Brake Switch A/B Correlation Code.” Any problem with the brake switch, which closes and activates the brake lights when the driver steps on the brake pedal, will cause on-board diagnostics (OBD) to log a P0504 code.
  • Causes of the P0504 code include a failed brake light switch, a brake pedal that’s out of adjustment, wiring issues, and powertrain control module (PCM) failure.
  • Symptoms of the P0504 code include shifting issues, the engine stalling when braking, inoperative cruise control, and brake lights that won’t illuminate.

The brake switch alerts the powertrain control module (PCM) that the brake pedal is depressed. The switch receives current from a constantly hot fuse and is normally open (N.O.) unless the driver steps on the brake pedal. If the PCM detects an incorrect signal or an electrical fault coming from the brake switch, it will log a P0504 code.

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How a Brake Switch Works

stepping on the brake pedal pad
As the driver steps on the brake pedal, a spring causes the brake switch to close which completes the electrical circuit and activates the brake lights.

As the driver steps on the brake pedal, a spring causes the brake switch to close which completes the electrical circuit and activates the brake lights. Once the pedal is released, a plunger connected to the switch is depressed and turns off the lights.

The brake switch is also used as an input switch for cruise control, anti-lock brakes, and brake shift interlock.

The brake and turn signal switches are not connected for systems using separate filaments for the stop and turn signal lights. If the vehicle uses one filament for both lights, brake switch current is routed through contacts within the turn signal switch.

What Does a P0504 Code Mean?

Diagnostic trouble code (DTC) stands for “Brake Switch A/B Correlation Code.”

auto mechanic using diagnostic scan tool inside the car
Any problem with the brake switch will cause on-board diagnostics (OBD) to log a P0504 code.

The brake light switch is a “normally open” switch held by a tab on the brake pedal. This keeps the brake lights turned off. Once the brakes are applied, the switch closes and activates the brake lights.

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Any problem with the brake switch will cause on-board diagnostics (OBD) to log a P0504 code.

Note: The definition of code P0504 might be different depending on the vehicle manufacturer. Consult the appropriate repair manual or repair database for the exact code definition.

What are the Common Causes of the P0504 Code?

What are the Common Symptoms of the P0504 Code?

How to Diagnose the P0504 Code

DTC P0504 is a generic powertrain code. Makes like Kia, Hyundai, and Toyota, can log a P0504 code. While vehicles can exhibit similar symptoms, steps for diagnosis and repair can vary depending on their specifications. If you’re not familiar with auto repair, it might be a good idea to bring your vehicle to a professional.

Here is a video that shows what the diagnostic process might involve:

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How to Fix the P0504 Code

Attempting to fix a P0504 code can be confusing if you don’t have the right tools and information. If you’re not confident with your DIY skills yet, it would be best to consult a certified mechanic. If you’re a seasoned DIYer who plans on resolving this code on your own, make sure to read your vehicle’s repair manual before proceeding. You can check out guides like those from Chilton or an ALLDATA subscription for vehicle-specific repair information before your start.

About The Authors
Written By Automotive and Tech Writers

The CarParts.com Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by CarParts.com's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Contact Center Manager and Technical Reviewer at CarParts.com

William “Bill” Guzenski has produced hundreds of how-to videos for the automotive community. He’s an ASE Certified Master Automobile Technician, and is affiliated with the Specialty Equipment Market Association (SEMA). He loves attending race events and car shows throughout the country, as well as traveling in his 40-foot motorhome, exploring abandoned mines and ghost towns.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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Patrick stuart

I love the DIY comments and videos it’s very helpful

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