My AC and heater unit works fine but the heat will not come on when the thermostat is set in the automatic position, The AC works like it should, turning off when temp gets to the set temp. The only way I can heat the house is leaving the fan on all the time.

Could be a defective thermostat, or a fault in the control wiring from the stat to the heating unit, or a fault with a control component in the heating unit.