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.
- The brake lights will either be illuminated continuously or fail to illuminate as the pedal is depressed
- It will be difficult or impossible to shift out of park
- The vehicle may stall when the brakes are depressed while a cruising speeds
- The cruise control will not activate
There are several components in this circuit, any of which is capable of compromising the circuit sufficiently to set this code.
- The most common is the brake light switch whose failure is due to wear.
- The brake light fuse will fail at times due to moisture in the circuit or burning out the brake light.
- Brake light bulb failure is another cause many times due to water infiltrating into the lens.
- The wiring harness, more specifically the connectors are loose or pushed out pins will cause a correlation problem between the switch and PCM.
- Lastly, the PCM itself could 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. 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.
- Have a helper check the brake lights. Make sure they operate by cycling them on and off and that the bulbs are good.
- If the brake lights are on continuously the brake light switch is either improperly adjusted or faulty. The same applies if they fail to operate. 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 pull off the connector.
- With the voltmeter check the voltage on the red wire in the connector. Place the black lead on any good ground and the red lead into the red wire terminal. You should have 12 volts, if not check the wiring to the fuse block.
- Plug the connector into the switch and backprobe the white wire with the pedal depressed. You should have 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 of the adjustable category, check the adjustment. The switch must be in good contact with the pedal arm and fully depressed.
- If the brake lights function properly yet you still have the code, heck the remaining wires on the brake light switch. Remove the connector and probe the remaining wires for power. Make note of the location of the power wire and replace the connector. Back robe the wire adjacent to the power wire with the pedal depressed. If there is no power replace the switch.
- If the last test had power as the pedal was depressed the switch is good. The problem exists in the wiring to the computer or with the computer itself.
- Locate the computer and back probe STP terminal on the computer to ground. If 12 volts is displayed on the voltmeter the computer is bad. If there was little or no voltage replace or repair the harness from the computer to the switch.
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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In this video I’ll be going over a brief description of what a p0504 brake switch a b correlation code means.
And what it took to fix the brake switch on this vehicle. Is a very common cold on a lot of Mazdas and Hyundais I’ll show you where the brake light switch is located and also how to check it.
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Ошибка P0504 – Brake Switch Circuit ‘A’ / ‘B’ Correlation
– Faulty stop lamp switch
– Blown stop lamp bulb
– Brake switch harness open or shorted
– Brake switch circuit poor electrical connection
– Горит индикатор неисправности двигателя "Chek Engine" или контрольная лампа "Service Engine Soon"
– Engine Light ON (or Service Engine Soon Warning Light)
In addition to turning on the stop lamp, the stop lamp switch signals are used for a variety of engine, transmission, and suspension functions as well as being an input for diagnostic checks. It is important that the switch operates properly, therefore this switch is designed with two complementary signal outputs: STP and ST1.
The Engine Control Module (ECM) analyzes these signal outputs to detect malfunctions in the stop lamp switch.