Furnace lights OK but the furnace fan will will not come on when it should in automatic mode, runs OK in manual mode. Thank You

I do not have the fix for you because its likely on the circuit board and I have no means to test. However, I have some great news for you. Save the money fixing this and just leave the fan in the ON position. It will use only a very, very small amount of extra electrcity but your home will be more comfortable and uniformly heated/ air conditioned. When I move into a home the first thing I do is move to stat to ON, it is not a waste of energy.

Hmm we had a furnace that would not come on, turned out to be one wire had touched another and had "fried" the circuit board. However, leaving it on should be just fine, you may just want to double check in the furnace that nothing is burning or shorting out. It could also be a simple thermostat problem; but I'm not qualified to tell you what to look for. You might take the cover off the thermostat and double check all the wires are connected. Hope this helped