The vehicle’s computer controls all circuits looking for problems in one of them, such as voltage deficiency or an out of range position.
If the Power Train Control Module (PCM) has set the DTC code P0504 OBDII it is because it has detected a recognized fault in the brake light circuit. Because the light brake switch is connected to several circuits, there are situations of risk. This switch has two signal outputs and if one of them is malfunctioning, this diagnostic code is established. This code must be attended in the shortest time possible for the safety of the vehicle.