I have a Nordyne Intertherm FEHB-015HA furnace. The circulation fan will not shut off. The thermostat works properly and kicks the elements off as it should, but the fan continues to run. There is a fan switch on this unit that can be set to auto or off, but it will not turn the fan off. The only way to shut the fan off, is to shut the whole furnace down. What could be the probable cause and any troubleshooting references would be appreciative.