Technical Description of OBD-II Trouble Code
Correlation Issue with Brake Switch A/B
This diagnostic trouble code (DTC) is a standard powertrain code that applies to vehicles equipped with OBD-II. While it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
What does that mean?
Your vehicle’s powertrain control module (PCM) has detected a failure in the brake light circuit, leading to the setting of the P0504 code. The PCM monitors all circuits for abnormalities such as voltage deficiencies or out-of-range conditions.
The brake light switch is connected to multiple circuits, all of which can result in a dangerous situation. The switch has two signal outputs, and if there is a malfunction, it will be detected and the code will be set. The cost of replacing the part and the labor involved are not significant. However, it is crucial to repair the issue as soon as possible due to safety concerns.
Symptoms
The brake lights may stay illuminated continuously or fail to illuminate when the pedal is depressed
Shifting out of park may become difficult or impossible
The vehicle may stall when the brakes are applied at cruising speeds
The cruise control may not activate
Potential Causes
There are several components in this circuit that can compromise it enough to trigger this code.
The most common cause is wear-related failure of the brake light switch.
The brake light fuse may fail due to moisture in the circuit or the brake light burning out.
Failure of the brake light bulb is another common cause, often due to water entering the lens.
Loose wiring harness connectors or pushed out pins can cause a correlation problem between the switch and PCM.
In rare cases, the PCM itself may have failed.
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. To access the switch, you need to move the front seat rearward, lie 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
Have a helper check the brake lights. Make sure they operate by cycling them on and off and that the bulbs are functioning properly.
If the brake lights are continuously on or fail to operate, the brake light switch may be improperly adjusted or faulty. To check, move the driver’s seat back and look under the dash. Squeeze the tabs on the electrical connector located on the brake light switch and disconnect it.
Using a voltmeter, check the voltage on the red wire in the connector. Place the black lead on a good ground and the red lead into the red wire terminal. You should measure 12 volts. If not, check the wiring to the fuse block.
Plug the connector back into the switch and backprobe the white wire with the pedal depressed. You should measure 12 volts with the pedal depressed and no voltage with the pedal extended. If there is no voltage, replace the brake light switch. If voltage exists at the white wire with the pedal extended, replace the switch.
If the switch is adjustable, check the adjustment. The switch must be in good contact with the pedal arm and fully depressed.
If the brake lights function properly but the code persists, check the remaining wires on the brake light switch. Remove the connector and probe the remaining wires for power. Take note of the location of the power wire and reconnect the connector. Backprobe the wire adjacent to the power wire with the pedal depressed. If there is no power, replace the switch.
If the last test showed power as the pedal was depressed, the switch is good. The problem lies in the wiring to the computer or with the computer itself.
Locate the computer and backprobe the STP terminal on the computer to ground. If the voltmeter displays 12 volts, the computer is faulty. If there is little or no voltage, replace or repair the harness from the computer to the switch.
Additional Notes
Please be aware that some vehicles are equipped with driver-side knee airbags. Exercise caution when working around airbags.
Here is an image of the brake pedal switch on a 2011 Ford F-150
