Check the control wiring at the thermastat and the main board. I'm guessing that since there are two boards on is for a heat pump and could possibly be attached to the side of the furnace. If this is the case check the one on the inside. Pay close attention to "G" to see if any other wire is touching this terminal.

Of course the first thing to chech is make sure the fan setting on the thermostat is set to auto.