This indicates that there is a ground fault on one of the attached SC-40C satellite controllers. The SC-40C that has the ground fault will have a solid red GFEP light and will not allow the heaters to be powered. Isolating the heater leg that has the ground fault is done in the same way as on the Isolating a ground fault on the APS-4C. If the light is flashing but there is no indication of a ground fault on the APS-40C then review the wiring of the communication wiring between the APS-4C and the SC-40C.