Diagnostic Trouble Code
P0504
Brake Switch A/B Correlation Code
Technical Description P0504
Brake Switch A/B Correlation
What Does This Mean P0504?
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.
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 P0504
1
The brake lights will either be illuminated continuously or fail to illuminate as the pedal is depressed
2
It will be difficult or impossible to shift out of park
3
The vehicle may stall when the brakes are depressed while a cruising speeds
4
The cruise control will not activate
Possible Causes P0504
1
The most common is the brake light switch whose failure is due to wear.
2
The brake light fuse will fail at times due to moisture in the circuit or burning out the brake light.
3
Brake light bulb failure is another cause many times due to water infiltrating into the lens.
4
The wiring harness, more specifically the connectors are loose or pushed out pins will cause a correlation problem between the switch and PCM.
5
Lastly, the PCM itself could have failed.
Solutions & Repair Steps P0504
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.
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
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.
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