My Daynight furnace ( product # 394JAZQ48065) fan runs constantly unless I unplug it the whole furnace. The facts below may be of help.

The display on my Honeywell thermostat went blank when it first started to happen. I replaced the batteries and it now reads "-AC". (While the fan simply runs.)

This happened once several years ago and all I did to fix it was remove the top heater panel and push a reset button that is located in the center of an angled down panel. That button has a wire leading in and one out. As I remembered it would click. I now push it and hear no click. Might that need to be replaced?

Once again when I plug the unit in the fan simple runs immediately.

I would really appreciate any advice that you could share.

Thanks
Mark

Look for a fuse on the furnace control board and then for a grounded wire in the 24 volt circuts