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P0504 is a diagnostic trouble code (DTC) for "Brake Switch “A”/”B” Correlation". This can happen for multiple reasons and a mechanic needs to diagnose the specific cause for this code to be triggered in your situation. Our certified mobile mechanics can come to your home or office to perform the Check Engine Light diagnostic for $79.99. Once we are able to diagnose the problem, you will be provided with an upfront quote for the recommended fix and receive $20.00 off as a credit towards the repair. All our repairs are backed by our 12-month / 12,000-mile warranty.
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.
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.
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:
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:
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.
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.
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.
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:
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.
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.