P0504 — Brake switch signals do not match or sync properly

The brake switch sends signals to the car’s computer about brake pedal position. This code appears when the signals from two brake switch circuits don’t match or correlate as expected. It can cause issues with brake lights, cruise control, or transmission shifting.

Common causes

The cause is almost always one of a few things. Here they are — P0504 rarely comes from anything outside this list:

• Faulty or worn brake light switch
• Damaged or corroded wiring/connectors at brake switch
• Misadjusted brake switch position
• Faulty brake pedal sensor or related component

Where to start

In the order a sensible mechanic would work — the cheap checks first, the expensive replacements only after:

1. Inspect and clean brake switch connectors and wiring
2. Check and adjust brake switch alignment and position
3. Test brake switch operation with a multimeter
4. Replace faulty brake light switch if needed

That is what holds for P0504 in general. From here it depends on your engine — tell me which car it is and what it does, and I will give you the steps for it: real procedures, torque figures and diagrams instead of general advice.