Got an HVAC issue with the furnace I figure at least one of you guys works in the trade or knows more about it than I do as the homeowner and could help out with some knowledge or advice. It might be a bit long so please bear with me. On my York 95.5 furnace the furnace fan was on but was getting no heat. Flame was off and the furnace impeller fan was also off. The magic eye was blinking 5 red LED. Looking it up that read "flame limit switch open circuit". Cycling the power, the impeller fan came one, then quickly turned off, igniter never glowed so (obviously) gas did not come on, and no flame. Main furnace fan stays on so unheated air coming out of the furnace outlets upstairs. I did some reading and saw a pretty good explanation on youtube. On mine one of the two causes listed was the evaporation tank pressure hose was clogged or bad switch. I cleaned out the hose it was filthy but that not it. Applying a bit of pressure hear the switch trip. The other and main reason for 5 red LED blinks was flame limit switch open. One of the furnace limit switches above the burner box was indeed tripped. It is a push to reset switch. I reset it. The furnace then cycled on, igniter lit, gas opened and I have heat. I looked at the flame and it seemed like nice cones not visibly backing up into the furnace which is the safety feature this thermal switch is supposed to catch. Yet sometime, today, the switch indeed tripped. The videos showed very wavy flames when there's a problem but mine are not like that. To be honest I would not know a good from bad flame but saw no flames backing up into the furnace. It also said a clogged exhaust could to it. Mine is a PVC that comes out the side of the house and is a two in one pipe, combination inlet and exhaust in a nested pipe. I did notice steam coming out of the exhaust today before I noticed the problem. I noticed cold air leaking out of some seams that were not already closed with the steel tape, which I have 1/2 a roll of so I sealed those. The video says it open switch could be a cracked or hole in the heat exchanger though which has me spooked due to possible CO backup into the house. The CO detectors did not go off. Surely if I wake up tomorrow in the same situation I will not reset it but it's a holiday weekend and I am getting heat now. Can this be a freak thing or a bad switch itself? Furnace is 7-8 years old. The other side did not trip (there are two of them left- and right. Also is some sort of furnace cleaning in order? Any advice welcome. Thanks in advance.