My true blue 90 Quatro furnace had a pressure switch replaced yesterday. (Consolidated industries USA 072 NH3R) After it was replaced, it would only fire with the door off the upper combustion area. The technician started checking for blockages in the exhaust and intake air. Funny thing, they found no blockages in either, but after they had cut into the exhaust line the sealed it back, the furnace wouldn't ignite even with the door off the combustion area. They are supposed to return today, but said they needed to research the problem. Could the problem be the heat exchanger? Are there any other small sensors or dirt issues that could cause this problem? Thanks for any answers. They were talking replacement furnace when they left...