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Fixing the Brake Switch Error Code P0504

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A brake switch controls the brake lights. The lights are turned on once the driver steps on the brake pedal with the help of an activated switch. The release of the pedal turns the lights off. But what happens when there is an error with the brake switch? This is when a P0504 error code is activated. It is important to fix this as an impaired brake could also lead to this error code. 

Car services

What is a P0504 code?

A P0504 code means that the engine control module has diagnosed a fault with the brake light switch signal control. It is also known as Brake Switch A/B Correlation.

Encountering this code is a reminder of a troubled brake pedal or the brake light control circuit. In other words, it indicates an unusual behavior when the brake light circuit is energized.

Symptoms of P0504 Code

If you come across these symptoms, one can certainly assume trouble with the brake light circuit and the associated systems.

  • Checking lights are turned on.
  • When the brake pedal is pressed, the brake light stays on or doesn’t turn on.
  • Stalling engine while applying brakes.
  • Irresponsive cruise control when brakes are applied.
  • Fault with the shift interlock safety system.
  • Being stuck in the Park mode even after switching on the ignition and pressing the brakes.

What causes this error code?

An inconsistent signal from the brake switch A/B correlation or the brake pedal sensors 1 and 2 is the primary reason for this error. It occurs when the ECU receives conflicting data regarding the position of the brake pedal.

The error code P0504 can occur due to issues with the brake light circuit and the multiple systems that are associated with it. However, this can also occur when there is an engine fuel mix or low brake fluid levels. The check engine light will remain on under the circumstances mentioned below.

  • Damaged brake light switch
  • Fault with the brake light fuse
  • Malfunction in the brake light bulb
  • Trouble in the circuit
  • Corroded or disconnected wiring linked to the brake light circuit
  • Corrosion in the brake light bulb holders

How serious is error code P0504?

This is a critical issue as it compromises the safety of not only the car which shows this but also of the passengers of the car that is behind it. A broken brake light puts the driver in a tough spot as it is impossible for the vehicles following it to determine whether or not the car is slowing down or coming to a complete stop.

Since the disengagement from cruise control even after applying brakes due to poses a dangerous situation, the error code must be taken seriously.


Meanwhile, if your car shows this error code, you are sure to fail an OBD-II emissions test. This is because a switched-off check engine light is mandatory to pass this test.

Therefore it is better not to overlook the symptoms for a long time and get it fixed as early as possible for the safety of the other drivers on the road as well.

How to diagnose error code P0504

The diagnosis will start with the inspection of the brake light switch, and the bulb which is the most likely cause.  The reading of all the stored codes and reading them.

Meanwhile, if no issue is observed with the brake light switch or the fuse, the diagnosis is automatically shifted to the wiring and the connectors. Since there is no fixed cause, the troubleshooting is continued till the reason for the error is detected.

How to fix error code P0504

There is no one-size fits all solution for the error code P0504 as the root cause can differ. However, the most common methods to fix this include:

  • Replacing the brake light bulb
  • Substituting the wires or connectors in the circuit
  • Brake switch replacement
  • Brake light fuse replacement

Can you fix the code by yourself?

The nature of the repair and replacement depends on the reason for the error. For instance, one can easily replace the limit switch. However, one must make sure to read the manual and understand it well before beginning the process.

To ensure safety, it is better to consult a certified mechanic since it can concern with the brakes as well.

How much does it cost to fix the P0504 code?

The labor charge for fixing this error can range anywhere between $95 and $125. However, the cost of repair and replacement varies with the cost of the parts that need to be substituted with new ones. For instance, the brake light switch can be replaced at the cost of $50 to $150. The cost varies based on the make and model of the vehicle as well.


Here’s more info on top-rated airport parking, the best parking spots in your city, the most affordable auto insurance for your car, and top-rated car washes near you.


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