Code P0504 (Meaning, Causes, FAQs)

And is it as critical as people say it is?

In this article, we’ll cover all you need to know about the P0504 code — its symptoms, causes, severity, and a convenient solution to it. We’ll also cover some FAQs to help give you a better perspective of diagnostic codes.

This Article Contains

What Is Code P0504?
What Does The P0504 Code Mean?
What Causes Code P0504?
What Are Code P0504 Symptoms?
Is The P0504 Code Critical?
How Is The P0504 Code Diagnosed?
How Is The P0504 Code Fixed?
What’s A Convenient Solution To Code P0504?
6 P0504 FAQs

Let’s roll.

What Is Code P0504?

The P0504 code is defined as “Brake Switch A/B Correlation” and is a generi c diagnostic trouble code (DTC) generated by the engine control module (ECM) of your car.

P0504 indicates that the ECM has detected a malfunction in the brake light switch signal circuit (stop lamp or stop light switch circuit).

What Does The P0504 Code Mean?

The ECM will highlight code P0504 if one of two situations occurs:

1. When the brake light switch itself fails. When this happens, the brake switch circuit will display some abnormality (like a lack of voltage or a signal that’s out of range). This alerts the ECM that there’s a malfunction in the brake light switch, so it sets the P0504 code.

2. The second situation is to do with any circuit that works with the brake light circuit (like cruise control or the shift interlock system). If these don’t respond as they should when the brake switch is activated, the ECM knows there’s a malfunction and sets the P0504 code.

FYI: The word “correlation” in the P0504 code description highlights a failure to correlate (or interact) with the brake light switch circuit.

Let’s look at the types of malfunctions that can trigger the P0504 code.

What Causes Code P0504?

The DTC P0504 may have several causes.

These can include a:

Brake light switch that’s failing from regular wear and tear (most common)
Blown brake light fuse (a damaged fuse can be a cause or a symptom)
Blown brake light bulb (possibly due to moisture)
A short or open circuit in the wiring harness from loose, broken, or bent connector pins
Pinched or chafed wire on the brake pedal that affects the electrical connection
A defective ECM (this is rare)

Now that you know what the causes are, what are the symptoms you can expect?

What Are Code P0504 Symptoms?

There may be more than one symptom occurring with the P0504 DTC.

Here are some common ones:

The Check Engine light turns on
A brake light either stays on, or doesn’t turn on at all, when the brake pedal is pressed
The vehicle stalls when the brake pedal is depressed at cruising control speeds
The cruise control system doesn’t respond when the brake pedal is activated
The shift interlock safety system doesn’t respond properly — it may be difficult to shift out of “Park” even with the brake pedal pressed, and the ignition switch turned on

Some symptoms, like the Check Engine light, don’t always mean it’s a brake light switch problem. The Check Engine light turns on for several reasons, including low brake fluid levels or engine fuel mix issues.

Now, you’ll likely be wondering how serious the problem is, right?

Is The P0504 Code Critical?

Yes. The P0504 is highly critical and should be attended to ASAP.

A malfunction in the brake light places the driver in a hazardous situation as the cars behind you can’t tell if you’re slowing down or making a sudden stop.

Don’t ignore the P0504 code.

Get it fixed immediately, and if possible, don’t drive to a workshop with it either.

Get a mechanic to come to you instead.

FYI: If the P0504 DTC causes the Check Engine light to turn on, your car could fail an OBD-II emissions test, even though the P0504 code has nothing to do with vehicle emissions. One of the prerequisites of passing this test is a Check Engine light that is off.

How Is The P0504 Code Diagnosed?

Your mechanic will switch the ignition on, read all stored codes, and clear the codes with their scan tool. They’ll conduct a visual inspection of likely causes, starting with the brake light fuse, then the brake light bulb.

If neither the fuse nor bulb displays any problems, they’ll move on to the brake light switch. They may have to refer to the manufacturer’s wiring diagram or manual to know which wire is which.

If there’s no issue with the brake light switch, the next step will be to eliminate the wiring harness, connectors, and so on.

This troubleshooting continues until the root cause is identified.

Once the culprit is located, the next step is to resolve the P0504 code.

How Is The P0504 Code Fixed?

Resolving a P0504 code depends on the root cause.

Repairs can involve:

Replacing a blown brake light bulb
Replacing a blown brake light fuse
Replacing a broken brake light switch
Repair or replacement of a damaged harness connector pins or wiring
Repair or replacement of engine control unit

But, what’s the best way to get the P0504 code fixed?

What’s A Convenient Solution To Code P0504?

The critical nature of the P0504 code means you need to get it carefully examined.

Luckily, this code is often fairly simple to fix.
With that said, you wouldn’t want to drive around with an unresolved P0504 code, even if it’s only to go to a repair shop. Getting a mechanic to come to you is a far better solution.

Lucky for you, that’s easy with AutoNation Mobile Service.

AutoNation Mobile Service is a convenient mobile vehicle maintenance and repair solution, and here’s why you should consider them:

Error code diagnoses and fixes can be made right in your driveway
Professional technicians execute the vehicle inspection and servicing
Online booking is convenient and easy
Competitive, upfront pricing
All maintenance and fixes are executed with high-quality tools and replacement parts
AutoNation Mobile Service offers a 12-month | 12,000-mile warranty for all repairs

How much will this likely cost?

For a code diagnosis, AutoNation Mobile Service typically charges between $95-$125, depending on your location. If you choose to continue with the necessary repair, the diagnosis fee will be applied to the final cost of the repair.

For example, replacing your brake light switch can cost anywhere from $50-$160. The total price depends on a few factors, including your location and vehicle make and model.

Fill this online form for an accurate estimate of how much your repair may cost.

Now that we understand the P0504 code a little better, let’s run through some FAQs.

6 P0504 FAQs

Here are some answers to FAQs you may be asking.

1. What Is A DTC?

DTC is the acronym for Diagnostic Trouble Code. It’s also referred to as an OBD code, fault code, or error code, and it’s generated by your vehicle’s onboard computer.

The DTC helps pinpoint the cause of any vehicle issues, so your mechanic knows where to start looking. For example, the P0571 or P0572 DTCs indicate problems with the cruise control switch.

Note: OBD stands for onboard diagnostics, and the current version is OBD-II.

2. What Is A Generic DTC?

A generic diagnostic trouble code reflects the same problem across any car installed with the OBD-II system, regardless of make and model.

3. What Is A Scan Tool?

An automotive scan tool is used to read and clear the DTCs generated by a vehicle’s onboard diagnostic computer. They may also be able to store, and playback live data, display pending codes, provide DTC definitions, and so on.

Some scan tools are specific to an automotive maker, like the Toyota Intelligent Tester for Toyota and Suzuki cars.

4. Where Is The Brake Light Switch Located?

The brake light switch (or stop lamp switch) is located beneath the dashboard, at the top of the brake pedal arm. Usually, the only way to access the stop lamp switch is to move the driver’s seat backward and look under the dashboard.

5. How Does The Brake Switch Work With The Brake Pedal?

The typical brake switch is a simple analog (ON/OFF) switch.

When the brake pedal is fully extended, the brake pedal arm depresses the brake light switch. This cuts off current, placing the brake switch in the OFF position.
When you depress the brake pedal, the brake pedal arm extends, turning ON the brake switch and activating the brake lights.

The brake switch assembly serves other functions, including deactivating the cruise control system and releasing the car from ‘Park.’

6. How Does The Brake Switch Circuit Work?

The engine control module (ECM), or powertrain control module (PCM), monitors the voltage in the brake switch circuit.

When you tap the brake pedal, the brake switch delivers a voltage signal to the ECM circuit. This voltage tells the ECM that the brake pedal is currently being pressed.
When you release the brake pedal, the brake switch circuit reconnects to ground. The lack of voltage then tells the engine control module that the brake pedal is free.

Concluding Thoughts

If the P0504 code pops up, don’t delay getting a mechanic to come look at your car. While it may be a fairly easy fix, the issue it presents is very critical.

Fortunately, AutoNation Mobile Service provides a quick solution to that, so contact them whenever something pops up, and expert mechanics will be at your doorway in no time to lend a hand!