[Nano]

Originally Posted by Delphinus

Here's something to try in the meantime (you have nothing to lose to try this .. maybe save yourself $200 in an emergency callout).

A lot of times problems with furnaces are dirty flame sensors. Open up the unit and look for what looks like a barbecue starter probe, a metal rod that goes to one of the burners. On mine it's on the far left burner. Undo this, take it out, and take a dry sponge or something midly abrasive and scrub that sucker. Maybe take some really really fine sandpaper if you see actual carbon scoring on it. But don't scratch it, you want the metal intact but any buildup or residue on it gone.

Put it back together and see if the furnace works properly after that.

Good luck!

+1 Rep point for you!