A flashing supply light indicates a missing or broken High-Temperature Limit sensor. This sensor is required to make the APS-C series controller to work. It is required on all of the APS-C series controllers.
The sensor is not polarized and must be connected to pins 10 and 11. Refer to figure 22 in the manual. If the Class two terminals are black with screw connections problems can occur when the screws are over tightened.
Above illustrates the terminal connections for the High Temperature sensor (thermistor) for the APS-4C, APS-3C, and the SC-40C Snow Switch Control Panels.