My dad is having a wiring problem with the thermostat. Once the heat is on it will not cut off using the thermostat. What does he need to do or buy to fix this problem. He has a 1992 model FEHA-015HA-01 serial number is FEH9201-00532

Is it a wiring problem? And where can he get the manual for this product.

Is it the original thermostat or did he put a new one on?

Does the furnace have a switch that reads Heat off Cool ?