P0504 Brake Switch “A”/”B” Correlation
OBD-II Trouble Code Technical Description
Brake Switch A/B Correlation
This diagnostic trouble code (DTC) is a generic powertrain code, which means that it applies to OBD-II equipped vehicles. Although generic, the specific repair steps may vary depending on make/model.
What does that mean?
Your vehicle’s powertrain control module (PCM) has set this P0504 code in response to a recognized failure in the brake light circuit. The vehicle’s computer monitors all circuits for abnormalities such as lack of voltage or an out of range condition.
The brake light switch is tied into several circuits all of which can result in a hazardous situation. The brake switch itself consists of two signal outputs and if there is a malfunction in the switch, it is detected and sets this code. This is not an expensive proposition in terms of the cost of the part or the labor involved in the replacement. The safety factor demands a repair as soon as possible.
Symptoms
Potential Causes
There are several components in this circuit, any of which is capable of compromising the circuit sufficiently to set this code.
Diagnostic Steps & Possible Solutions
The brake light switch is located under the dash at the top of the brake pedal arm. The brake booster lifts the pedal to its fully extended position. The brake light switch is mounted on a cross-beam support brace directly behind the brake pedal mounting bracket. The only method to access the switch is to move the front seat rearward, lay on your back, and look up under the dash. You will see a bracket housing a switch near the top of the brake pedal arm. The switch will have four or six wires.
The switch rests in the bracket so that its activating rod is in contact with the brake pedal arm when the pedal is fully extended. At this point the switch is depressed by the brake pedal arm cutting off the current. When the brake pedal is depressed, the arm extends turning the switch on and activating the brake lights. When the pedal is released, the arm again depresses the rod turning off the brake lights.
Diagnostic Steps
Additional Notes
Please be aware that some vehicles are equipped with driver side knee airbags. So be cautious when working around air bags.
Here is the brake pedal switch shown on a 2011 Ford F-150
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"Delete Error Code Permanently, Disable DTC Error Code Function for p0504 in the ECU"
In extreme cases, we even have the option to permanently remove the specific DTC p0504 and disable the function of the specific error code in the ECU. All other fault code functions will then continue to work to protect the engine. This is different from clearing an error code with a diagnostic device. We have the option to permanently remove the function of the specific error code, so that it never comes back. Also emergency running that occurs on the specific error code can be solved in most cases.
Of course we prefer to first try to solve the cause of the error code, but sometimes the only solution is to permanently disable the DTC function of p0504.