I have never worked on a furnace but my furnace is not heating i opened it and checked everything and everything is operating well non of the intake or exhaust piping is cloged up its clean.Both fan and blower motor working all the wiring looks good my best guess is that the the gas valve is not working well it looks like an electrical valve and it has a green light that blinks all the time but i think that when it was working fine it used to just be on all the time. Its a Armstrong air advantage 80 Furnace any suggestions greatly apreciated.