P0504 Error Code: Brake Light Switch “A”/“B” Correlation

A brake light switch is an electrical switch that turns the brake light when you depress the brake pedal.

It is an essential braking system component that notifies the driver behind you that you’re slowing down or putting the vehicle to a stop when you step on the brake pedal. It also works in harmony with the cruise control system, ABS module, and more.

If this switch malfunctions, the Powertrain control module will register trouble code P0504 to notify the technician of an imminent issue.

In this article, we’ll discuss the meaning, causes, symptoms, diagnostics mistakes, and how to correct this issue. Sit back, and have a 5mins read.

Code P0504 Definition

Brake Light Switch “A”/” B” Correlation

What Does P0504 Mean?

The error code P0504 refers to a correlation in a vehicle brake light switch. The Engine control module [ECM] will log in to this code whenever it detects any of the following situations.

In case, the brake light switch cannot interact as it should. If this happens, the brake switch circuit will send an out-of-range or low voltage signal to the ECM, alerting it to register diagnostics trouble code P0504.

The next situation is when there’s a malfunction to other system components that work in harmony with the brake switch, such as the shift interlock and cruise control system. If these components cannot respond as they should when the brake switch is activated, the car computer will know there’s a malfunction and set the error code P0504.

The word ‘correlation’ in the definition above means the inability to interact with the brake light switch circuit.

p0504 ford

What Are the Symptoms of Code P0504?

As with other diagnostics trouble codes, P0504 Toyota and other vehicles trigger the engine warning light to illuminate on the dashboard. Here are several other potential symptoms you should watch out for.

  • Check engine light
  • Shift interlock system malfunction
  • Hard or impossible to shift out of park position
  • Vehicle stalls at cruising speed when you depress the brake pedal
  • The brake light stays on continually or cannot turn on when you step on the brake pedal.

Symptoms like check engine lights don’t always point to brake light switch problems. Several issues, like engine misfiring and other trouble codes, can cause the ECM to trigger and illuminate the check engine light.

Now, let’s explore the causes of P0504 in these few paragraphs.

What Cause of Code P0504?

Several factors can cause the ECM to register this code, but luckily, it is easy to diagnose. The causes include;

  • Malfunctioning brake light switch
  • Electrical connectivity issues or brake switch circuit problems
  • Defective brake light switch
  • Faulty or blown brake light fuse
  • Short/open brake switch wiring harness
  • Blown brake light bulbs.

How Serious Is Code P0504?

Code P0504 is a severe case. It is perilous if the brake light doesn’t switch on or off when depressing or releasing the brake pedal. Drivers behind you cannot determine if you’re slowing down or cruising at the same speed, especially at nighttime.

Again, if the cruise control system cannot disengage when depressing the brake pedal, it may lead to a collision. This shows how severe this code is; hence, you need to consult your mechanic for an immediate fix.

Code P0504 Common Diagnosis Mistakes

Most times, when a brake light cannot light on when a driver steps on the brake pedal, he assumes the culprit is a blown brake light bulb. The same applies to mechanics. The technician may then replace the brake light bulb and find out it doesn’t fix the problem. Again, replacing a blown brake fuse with an underlying brake circuit or switch issue will not fix the problem, as the culprit may blow a fuse again.

Tools Needed to Diagnose Code P0504

  • OBD2 scan tool
  • Digital Multimeter

How to Diagnose and Repair Code P0504

Whether you have a P0504 Mazda or P0505 Volvo, you can diagnose and fix the underlying problem by following the guides below.

After pulling error code P0504 by plugging in the OBD2 scan tool via the vehicle onboard computing access port, clear the trouble code and inspect all potential causes.

Start by testing the brake light fuse and then head on to check the brake light bulb and replace it if necessary. If these do not provide solutions, inspect the brake switch itself and the electrical wiring harness and connectors.


Often, fixing the P0504 error code is pretty straightforward. Depending on the culprit, the possible repairs include;

  • Repairing or replacing wiring harness or connectors that leads to the brake switch or circuit
  • Replacing a blown brake light fuse
  • Replacing bad brake light bulbs
  • Replacing the brake switch.

Additional Comments Regarding Trouble code P0504

Aside from dangerously colliding with the road, P0504 can lead to a failed emission test. Indeed, a defective brake light switch does not directly affect a smog test, but it can trigger an engine warning light and cause the vehicle to fail OBD-II emission inspection.

Same Problems with different Error Codes

Here are other diagnostics trouble codes [DTC] that refer to the same or related issues with fault code P0504;

  • P0571
  • P0572



How much does it cost to replace a brake switch?

A brake switch replacement costs around $30 to $75, depending on your car’s make and model. However, if you choose to have an auto mechanic fix it, budget around $80 for both parts and labor.

What is the work of a brake pedal position sensor?

The brake pedal position sensor regulates or monitors the position or movement of a brake pedal to turn on the brake lights and brakes switch off the cruise control system when you step on the pedal.

What happens if a brake switch is bad?

A defective brake switch will cause a couple of drivability issues. If the switch fails, the brake light won’t come on when you depress the brake pedal, and your automatic transmission will be stuck in the park position. A lousy brake switch in modern vehicles with a push-start button feature will prevent the car from starting.

The brake lights may malfunction. It may come on without stepping on the pedal or refuse to come on when the driver depresses the brake pedal. It can also cause the check engine light and the ABS light to pop up on the dashboard. Sometimes, the brake light will stay on continually.

Final Thoughts

Whether you have a P0504 Range Rover Sport or P0504 Ford error code, either one is a severe case that requires urgent attention. If your vehicle ECM registers the P0504 code, don’t delay in contacting your mechanic to track down the culprit and fix it. While the repair may be easy, the effects it projects may endanger the life of the driver and that of other road users.

Whenever a check engine light pops on your dashboard, drive to a nearby local mechanic or contact your dealership for a professional diagnosis and fix.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

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