Go figure on a 3-degree morning my trusty old Lennox gas heater decided it didn't want to heat. With all the fix-its I do, never ventured much into furnaces. But damn, I figured calling a furnace guy on a one of the first coldest days and on a Saturday morning, would be a long wait. So ventured through the web viewing a dozen FAQ's, and videos with a search with the symptoms - furnace fan turns on immediately; but not heat. Well, fan shouldn't turn on instantly anyway. As you versed guys know, could be a bunch of things and was thinking ignitor, flame sensor, etc. I came across a young guy on a video describing my symptoms and he had a nifty trick: Tap on the high-limit sensor as it can be stuck open before getting into voltage testing, etc. I wasn't optimistic as most times with me the simple things are fine...so turned off power to be safe tapped on the switch a few times and turned furnace on. I noticed a green light on the circuit board...green is good...and what da hey! It's working. Could be might need a new one since it stuck once; but for now will enjoy da heat. Hope this trick might be handy if yaz get no heat.