The Mitsubishi Electric PAC-SK52ST Service Tool is a diagnostic accessory designed for Mitsubishi Electric P-Series outdoor HVAC units and compatible M-Series branch box systems. It connects to the CNM connector on the control board and uses DIP-switch settings to display operating data and self-diagnosis codes through its LED display. https://www.partshnc.com/pac-sk52st-mitsubishi-electric-service-tool