I have the same unit in my 318SAB and, yes, it is a prickly little beast. I assume that your is a little white 3" square box.
My unit will not go below 68° in the A/C mode, in the heating mode it will go lower.
When you set the unit to "FAN" you can use the +/- keys to change some settings.
DIF: allows you to set a temperature turn differential to either 2°*(default) or 1° for closer control.
FAN: sets fan mode. AUto (default mode) runs the A/C fan on High when you have a set/ambient difference of 5(?) degrees and Low when the differential is less. If you set to either HI or LO the fan is locked when the A/C is running.
Another (feature??): When fan is set in either the HI or LO mode and HEAT is called for, the A/C fan starts up along with the furnace. ONLY the fan, not the compressor. Confused yet? So was I and the dealer couldn't shed any light on it either. The thermostat manual was opaque on this point also.
If you have wither the Heat pump feature or an Aux electric heating element in the A/C unit, I think you need another model Thermostat.
I hope that this helps,