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P0504 OBD-II Trouble Code: Brake Switch “A”/”B” Correlation

Check Engine Light

Diagnostic Trouble Code (DTC): P0504

P0504 code definition

When a malfunction is detected in the brake light switch of a vehicle, the PCM (powertrain control module) records the P0504 code, and the Check Engine Light is illuminated.

What the P0504 code means

If a failure occurs in the circuit for the brake light, a number of problems can occur. To keep you out of danger this code will be recorded, and the Check Engine Light will remain on until the cause of the problem is resolved and the code is cleared from the PCM.

What causes the P0504 code?

A number of issues can cause the P0504 code to be recorded, but fortunately most of them are fairly simple and easy to diagnose. These include:

  • A blown or malfunctioning brake light fuse
  • A damaged or faulty brake light switch
  • A blown brake light bulb
  • A shorted or open brake switch harness
  • A problem with the electrical connection or circuit for the brake switch

What are the symptoms of the P0504 code?

The first indication that your PCM has stored a P0504 code will likely be an illuminated Check Engine Light. Aside from this, you may also notice other symptoms, including:

  • Pressing the brake pedal doesn’t engage or disengage the vehicle’s cruise control system.
  • One or both brake lights will not turn on when pressing the brake pedal.
  • One or both brake lights remain on, even after taking your foot off of the brake pedal.
  • Depressing the brake pedal at highway speeds causes the engine to stall.
  • The shift interlock system fails to work properly.

How does a mechanic diagnose the P0504 code?

After identifying the P0504 code with an OBD-II scanner, the mechanic will clear the code and then perform an inspection of all possible causes for the code. They will start by testing the brake light fuse, and then they will test and/or replace the brake light bulb. If these measures do not fix the problem, they will move on to test the brake switch itself, as well as its wiring and connectors.

Common mistakes when diagnosing the P0504 code

Often, when a brake light does not turn on when the driver depresses the brake pedal, they will assume that the problem is a burnt out bulb. They may then replace the bulb and find that this does nothing to solve the problem. If there is an underlying problem with the brake switch or circuit, replacing a blown brake fuse may also be a mistake, as the root problem will likely cause the fuse to blow again.

How serious is the P0504 code?

It’s very dangerous when brake lights do not turn on and off when depressing or releasing the brake pedal. Following traffic cannot tell if you intend to slow down or if you need to stop suddenly, and an accident can easily occur. Likewise, if depressing the brake pedal does not disengage the cruise control system, you may find yourself in another dangerous situation. Thus you can see that the P0504 code is very serious, and should be dealt with immediately.

What repairs can fix the P0504 code?

In most cases, repairing the cause of a P0504 code is fairly simple. Depending on what the root problem is, some of the most common repairs include:

  • Replacing a burnt out brake light bulb.
  • Repairing or replacing wires or connectors for the brake switch harness or circuit.
  • Replacing the brake switch.
  • Replacing a blown brake light fuse.

Additional comments for consideration regarding the P0504 code

In addition to causing potentially dangerous situations on the road, a P0504 code can also result in a failed emissions test. Though the brake light switch does not have a direct effect on the vehicle’s emissions, it will illuminate the Check Engine Light, which will cause the vehicle to fail an OBD-II emissions test.

Need help with a P0504 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
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