Can't get furnace to run in FAN ONLY mode
I notice that you have two red wires used for R and W.
Perhaps they have been swapped, so the wire labeled &quot;R&quot; at the thermostat is actually connected to &quot;W&quot; on the furnace control board, and vice versa. That would mean that when the thermostat tries to activate the fan, it is just connecting the wires for heat and fan, without connecting the power from R to anything, and so nothing happens.
If so, then you just need to swap back the two red wires and the fan control should work properly.