I've been struggling with the same problem for years. 3 blinking lights, reset the furnace, replaced the pressure switch, checked for leaks, water, air, everything. Just had a repair guy in today. Everything checks out. Good man. As we were winding things down, he mentioned that a strong gust of wind, in just the right direction, could trip the pressure switch and shut down the furnace. There was no hesitation on my part in answering him. "Every time this has happened, there has been a strong wind." He said, instead of removing and replacing the hose from the front of the pressure switch, all I had to do was reset the furnace, meaning, just turn it off and on at the switch. So folks, it's not your pressure switch, it's not water, it's not your heat exchanger, no, it's not the logic board. It's simply one thing, WIND!