I have a Lennox G40 Series furnace that keeps blowing the 5 amp fuse on the circuit board. Unit will come on, burners will ignite, blower comes on & 5 min later, "pop".
I've replaced the programmable thermostat to no avail. Put the old original thermostat back on that didn't work either. However when I take the thermostat out of the picture and put a jumper on the unit itself to make it run, it works fine.
This unit is about 3-4 yrs old. Could the circuit board have gone bad??

I would check the low voltage wire from the furnace to the thermostat for a short circuit. You answered your own question when you said you disconnected the thermostat and jumpered to unit itself and it works fine. Something in the wiring to that thermostat seems to be the problem.