The York S1-6421651 2 Stage Circuit Board is an electronic control component designed to manage the operation of compatible York two-stage HVAC systems. It processes signals from thermostats and system sensors to control heating, cooling, blower, and staging functions for efficient equipment performance. Commonly used in furnaces, air handlers, and packaged heating and cooling units, ... https://www.partshnc.com/s1-6421651-york-2-stage-circuit-board