Fault code P0504 – a step-by-step guide to diagnosing and fixing it

Error code P0504 is displayed and the check engine light is on - what does this mean?

The DTC P0504 is triggered by the OBD/EOBD system when a correlation is detected between the brake light switch A and B that is outside the expected parameters. This fault code affects the brake light and cruise control system and may have slightly different definitions for different manufacturers.

What causes can this error code have?

• Defective brake light switch
• Faulty wiring to the brake light switch
• Corroded or loose plug connections
• Damaged brake pedal or incorrect adjustment
• Defective brake lights or burnt-out bulbs
• Problems with the power supply to the brake light circuit
• Faulty ECU or control unit programming

What symptoms occur in parallel with fault code P0504?

• Brake light problems: The brake lights are not working properly or are permanently illuminated. Cruise control failure: The cruise control no longer works or switches off unexpectedly. Shifting problems: Automatic transmissions may have difficulty shifting out of park. Starting problems: In some cases, the vehicle will not start if the brake pedal is not detected. ABS warnings: Additional warning lights on the dashboard may illuminate.

How to fix the problem of error code P0504?

• The repair requires a systematic diagnosis of the brake light circuit and its components.
• Replace the brake light switch: Replace the defective brake light switch, which is usually located on the brake pedal, and adjust it correctly according to the manufacturer's specifications.
• Check wiring and plug connections: Check all cables to the brake light switch for damage, clean corroded contacts and ensure tight connections.
• Check brake lights and fuses: Replace defective bulbs and check all relevant fuses in the brake light circuit for continuity.
• Adjust the brake pedal: Adjust the correct position and distance of the brake pedal to the switch to ensure proper activation.
• Reset control unit: Clear the trouble code after repair with an OBD scanner and perform a test drive to verify function.