If it's snowing and the outside temp is lower than 38F, the SCP-120 should call for heat. If it's not calling for heat, it could be a problem with the controller or the aerial sensor.
Disconnect the aerial sensor from terminals 1, 2 and 3 and measure the voltage between terminals 2 and 3. Make sure the meter is set to VDC, not VAC. The voltage between terminals 2 and 3 should be 24 Vdc. If the voltage is about 10 Vdc, then they need to change the fuse behind the wire terminals. The fuse is 250 V, 2 Amps glass fuse.
If they have 24 Vdc between terminals 2 and 3, they need to put a jumper wire between terminals 1 and 2 and see if the Snow indication LED is on and if the SCP-120 is calling for heat. If it is then the problem should be in the aerial sensor. If it's not calling for heat the problem should be the SCP-120.