I would check behind the thermostat first for loose wires in the wall (or perhaps you have a defective thermostat?). My AC didnt have a Low speed for the fan but I found the wire beind the thermostat was disconnected. Perhaps the AC wire is shorting out right beind the Thermostat? Next take the cover off (on the inside) and look for a loose wires?
I dont know anything about AC systems but I would look for loose wires before I brought mine in to be serviced.
Hopefully someone else can chime in that had this same problem.
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