|Trouble Code||Fault Location||Probable Cause|
|P0504||Brake switch -A/B correlation||Wiring, mechanical fault|
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What Does Code P0504 Mean?
OBD II fault code P0504 is a generic code that is defined as “Brake switch -A/B correlation”, and is set when the PCM (Powertrain Control Module) detects a failure either in the brake light control circuit, or when one or more systems that are coupled to the brake light circuit do not activate/deactivate when the brake light switch is activated by pressing the brake pedal. Note that in the context of this code, the word “correlation” refers to whether or not an associated system reacts when the brake light circuit is energized.
On most applications, the brake light switch is a simple analogue (ON/OFF) switch that is normally open when the brake pedal is in the fully extended position. When the brake pedal is depressed, the pedal acts on the switch, which closes a circuit, causing the brake lights to illuminate.
However, the brake light circuit is tied into several other circuits/systems, with the cruise control and shift interlock safety systems being the most common. In practice, if the cruise control system is in operation, applying the brakes will both illuminate the brake lights momentarily, and deactivate the cruise control system. Similarly, on some, if not most applications it may be impossible to move the gear selector from “P” or “N” unless the brake light circuit is activated.
Thus, if the PCM detects that an associated system is energized/activated/deactivated along with the brake light circuit, a “failure to correlate” situation is recognized, and it will set code P0504 as a result, and may illuminate a warning light as well.
The image below shows a typical brake light switch, as well as its typical location. Note though that access to the switch for testing purposes can be problematic, and in some cases, parts of the dashboard and even seats have to be removed to get to the switch comfortably.
What are the common causes of code P0504 ?
Some common causes of could P0504 could include the following-
- Defective brake light switch (By far the most common cause)
- Damaged, burnt, shorted, disconnected, or corroded wiring/connectors in any system that is associated with, or tied into the brake light circuit
- Blown fuses/fusible links
- Blown brake light bulbs
- Corroded brake light bulb holders
- Failed or failing PCM. Note that this is a rare event, and the fault must be sought elsewhere before any control module is replaced
What are the symptoms of code P0504 ?
Some common symptoms of code P0504 could include the following-
- Stored trouble code and possibly an illuminated warning light
- Additional codes relating to the cruise control or other systems may be present
- Brake lights may be on permanently, or fail to illuminate
- Gear selector may be impossible to move out of “P” or “N”
- Cruise control system may not activate, or fail to deactivate
- On some applications, the engine may stall unexpectedly if the brakes are applied at cruising speeds
How do you troubleshoot code P0504 ?
WARNING: While diagnosing and repairing code P0504 is generally easy and inexpensive, the fact that this code is present poses a serious risk to life and limb. Therefore, this code should NOT be ignored; in fact, it must be resolved at the first opportunity that presents itself to avoid a possible crash.
Record all fault codes present, as well as all available freeze frame data. This information could be of use should an intermittent fault be diagnosed later on.
Refer to the manual to locate the brake light switch. Once it is accessible, refer to the manual to determine which terminals are power outputs and note the color coding of these wires. Also determine the input wire, and check that the specified voltage is present. If the input voltage is abnormal or absent, inspect the related fuse, but do NOT replace it if is blown until it certain that the short circuit that had caused the fuse to blow is repaired.
If input power is present (and correct), operate the brake switch manually while an assistant monitors the brake lights. If the brake lights illuminate, check that the switch is properly adjusted, and correct its adjustment until the brake lights illuminate when the brake pedal is depressed.
NOTE #1: If the brake lights do not illuminate when the switch is operated manually, but the related fuse is intact check and replace the brake light bulbs if required.
NOTE #2: If the fuse is intact and it is certain the brake light bulbs are in working order but the brake lights do not illuminate, check that the switch actually passes a current into the brake light circuit when the switch is operated manually. Note that this current is usually equal to battery voltage. If the switch passes the correct current but the brake lights still do not illuminate, check the brake light circuit for open circuits, and make repairs as required.
If the brake light switch does not pass a current into the brake light circuit, or for that matter, into any power output terminal or circuit, the switch is defective and it must be replaced with an OEM replacement to ensure proper operation and a reasonable service life.
NOTE: If the brake light switch is replaced on applications where adjustment of the switch is required, be sure to check that the switch is properly adjusted to ensure proper operation of the switch.
If the switch passes the correct current into all associated power outputs/terminals but the code persists, refer to the manual to determine the correct testing procedures for all associated circuits, since faults in associated circuits/systems can also cause code P0504 to set. Note that testing procedures for associated systems are make and model specific, so be sure to follow the directions in the manual exactly to not only obtain the most accurate test results, but to avoid misdiagnoses and the unnecessary replacement of parts and components.
Be aware that if the brake light switch/circuit checks out, it is entirely possible that the fault may be buried deep in the CAN (Controller Area Network) bus system, which is a highly complex conglomeration of wiring harnesses that interconnect all, or most of the control modules on the application.
If this is suspected, the wiser option is to refer the vehicle to the dealer or other competent repair facility for professional diagnosis and repair, since diagnosing faults in the CAN bus system invariably requires advanced diagnostic equipment and skills.
Codes Related to P0504
There are no known codes that are directly related to P0504 – “Brake switch -A/B correlation”.