DTC P0504: Malfunction of the Stoplight Switch System: Operation

WARNING: This page is about a different car, the 2014 Mitsubishi Outlander. However, it is still accessible from the selected car via links, so may be relevant.

For the stoplight switch, two switches, a stoplight switch for the stoplight illumination and a brake switch exclusively for the cruise control system, are incorporated to improve the reliability.
As for the stoplight switch, when the brake pedal is depressed/released, the stoplight switch ON/OFF signal is transmitted from the ETACS-ECU to the ECM via CAN bus line.
As for the brake switch, the ECM connector terminal BRK monitor the state of the brake switch. The brake switch turn ON/OFF when the brake pedal is depressed/released, and the input signal to the ECM connector terminal BRK changes. According to this change, the ECM judges the state of the brake switch.