I was changing my thermostat and I attempted to make the swap live without turning power off. When I connected new thermostat, the fan kept running.
Ruled out bad thermostat and wire short by disconnecting those wires from the board and turning furnace on, fan still turned on.
Ruled out bad board by buy a brand new one. Fan still runs
Ruled out burner buy unplugging it from the board fan still ran.
Look at wire lay out and the fan has 4 cables, one to a on/off low speed fan switch, one to a number "2", one to a "cool", and one to "heat" I tested it by unplugging the "cool" and fan still turned on. I then unplugged "heat" and the fan didn't run. I then left heat unplugged and tested thermostat and it worked like it should, turning the fan on and off when I told it to.
As far as I can tell. The "heat" cable goes directly from fan motor to the board. What could cause the fan to run?