Furnace won't light or only last few seconds
American standard furnace. The original problem was the furnace wasn't lighting, it would sit there and the element would light, but only on occasion it would light with gas. When it did... it would die shortly after. Come to find out from an HVAC specialist, a blow dryer pushed on the mainboard (get it warm, not hot) will change the characteristics of the signal paths. Made it work reliably. So I replaced the board and the last 2 years plus (since 2015) haven't had another problem.