Have a Lennox heat pump that ices up on me. Have replaced defrost board & checked out refg charge. There does not seem to be a sensor connected to board, and the wiring diagram is un-readable. Fan speed is OK, reversing valve is OK. Can not get it to go into defrost cycle by itself. Need advise & a wiring diagrtam for Lennox HP HP22-411-3P. Also does anyone know what manufacter handles parts for Air Pro heat pump, can not find any in my area.