P0504: what it means, why it occurs

Diagnostic code P0504 is an inconsistency between signals of Brake Pedal Sensors 1 and 2 or Brake Switch A/B Correlation. It appears when the ECU receives inconsistent information about the current position of the brake pedal.

More often error P0504 is fixed in Renault (Logan, Duster), Toyota (Corolla, Land Cruiser Prado) and other models in which this information is transmitted to the controller from the four-pin switch of brake lights.

When code DTC P0504 appears

DTC P0504 is caused by certain signals from the brake light limit switch received by the electronic control unit. In a number of cars in its body are combined two pairs of contacts: normally closed — the brake pedal position and normally open — brake light circuit. When the pedal is depressed, the first contact opens and the second contact closes.

If they do not correspond to each other, for example, both contacts are closed or open, or the accelerator pedal is depressed simultaneously with the information about the depressed brake pedal, the error P0504 occurs.

Error symptoms

P0504 Brake Switch A/B Correlation DTC «How to Fix»: video

Since the repeated discrepancy between the signals from sensors of brake and gas pedal pedals position can cause the engine to go into the emergency mode the problem is defined by error code P0504 and may be manifested by the following symptoms:

• The brake light illuminates continuously or does not illuminate when the brake pedal is depressed.
• Cruise control does not activate.
• Dynamic performance is impaired, engine rpm does not rise above 1250-1500 per minute and/or drop to idle on acceleration, the engine shuts down or does not start.
• On cars with an automatic transmission, there may be problems with changing the selector position.
• The Check Engine indicator on the dashboard lights up (usually when the error is repeatedly fixed).

Causes of mismatch of brake sensor signals and their elimination

In most cases, error P0504 occurs due to a brake light limit switch malfunction or its improper adjustment (if required). However, the failure is also possible in case of burnout or installation of non-standard lamps in the rear lights. The main causes of mismatching brake sensor signals and how to correct them are discussed in the table below.

After removing the cause of malfunction it is necessary to reset error P0504 with a diagnostic scanner and special application, otherwise the engine may continue to run in the emergency mode.

Diagnostic procedure when error P0504 appears

To perform some diagnostic operations when error P0504 appears, you need a multimeter or a test lamp. If you do not have an assistant to check the operation of the limit switch and the condition of the brake light lamps, you can drive up to the wall so that the brake light can be seen in the mirrors.

How To Replace Brake Pedal Position Switch: video

1. Check the brake lights by depressing and releasing the brake pedal several times. The lights should illuminate when the brake pedal is depressed 2-3 cm and go out when the brake pedal is released. Blinking or flickering indicates poor contact, and intermittent failure to operate indicates improper adjustment or wear of the limit switch.
2. If you get error P0504 after replacing the brake light bulbs with LED ones, first of all try to install the original halogen ones back. This is most likely the problem.If the lights do not light when the pedal is depressed, make sure the brake light circuit fuse is not blown.
3. Inspect the limit switch connector, make sure it is secure and there is no external damage.
4. Check the brake light limit switch and its adjustment. Refer to the next section for details on how to check it.
5. Check the power supply to the sensor with a multimeter or test lamp. One of the pins must be permanently energized at 12 V. Use the vehicle wiring diagram to determine the appropriate contact. If there is no power, check the corresponding circuit in series.
6. If there is power and the sensor is functioning, check the signals with the pedal depressed and released on the appropriate ECU pins with a multimeter. Their presence indicates a malfunction in the controller.

Checking, Repairing and Adjusting the Vehicle Stop Light Switch

limit switch wiring diagram

Inside the four-pin brake switch there are two pairs of contacts, the state of which depends on the position of the rod: when fully recessed and fully extended, one pair is always closed — resistance is close to zero, the other is open — resistance is several kOhm or more. If for some reason the contact group does not work properly, error P0504 occurs.

Checking the brake light switch

Inspection of the brake light switch is reduced to the testing of the pins in the normal position — with the rod extended and retracted, simulating the brake pedal depressed. Normal state of the contacts of the switch for some cars may be different, but for example on the Toyota Rav4 the principle is as shown in the table below.

If the check shows that the contacts close and open properly, error P0504 may be related either to improper adjustment of the brake light switch (not required on all models) or to faulty wiring and other above-mentioned reasons. If the multimeter readings do not match the parameters of a functioning switch, it must be replaced or repaired.

How can you fix the problem without buying a new part?

The two main causes of brake light limit switch failure are oxidized or burned contacts and broken or loose spring. An indirect sign of the second problem is a short stroke of the rod compared to a serviceable one or its free movement.

To repair the limit switch it is necessary to dismantle it. To do this first of all it is necessary to disconnect the connector by releasing the lock. In some cars for this purpose it is necessary to unscrew the fastening nut, in others, it is enough to turn its body counterclockwise by 90 degrees and remove it from the hole.

Next, you should disassemble the sensor housing, which is fastened on the latches, and then examine the condition of the spring and contacts. Oxidized or burnt contacts can be cleaned with sandpaper and treated with degreaser. Broken spring must be replaced by a new one of the same length, and you can try to stretch the weakened spring, but it will help only for a while.

What to do if the problem cannot be solved without replacing the part?

If it is not possible to repair the limit switch, you can order it on ebay and replace it yourself, it is not difficult, but in most cases you may need to adjust it on the pedal.

When the part is attached to the bracket of the brake pedal with a pair of nuts, the correct position is when the gap between the tip of the rod and the body of the switch does not exceed 0.5-1 mm when the brake pedal is not depressed. With a larger gap, the impact of a special flush on the pedal is not enough to sink the stem until the corresponding contacts close and open.

The new type switches that lock onto the bracket without a nut (removed by turning the housing) are self-adjusting. Before installing them, it is sufficient to extend the ribbed rod, which itself will take the correct position after installation. The limit switch has a mushroom cap that protrudes as far as possible. After installing the limit switch, you have to slowly release the pedal so that it comes back up. This way, the pedal will snap back as many divisions as it needs to.