The pressure switch stays together. I have to blow in line to get switch to release contact from each other then it will fire up. What causes that and what makes the furnace open the pressure switch?

Pressure switches do two jobs. They tell the control board that the flue gas purge blower is running and check for blockages in the flue and intake pipes. (If you have an intake pipe) . They can also be used as safety switches and can detect cracks in heat exchangers that could allow carbon-monoxide into the air stream blowing through it. You could have a bad pressure switch or a bad hose to it. A pin hole or crack in the hose could cause your problem. A blocked intake or flue pipe or cracked heat exchanger . Check your hoses and pipes, if no problem there, I'd have it inspected by a professonal.

Pressure switches do two jobs. They tell the control board that the flue gas purge blower is running and check for blockages in the flue and intake pipes. (If you have an intake pipe) . They can also be used as safety switches and can detect cracks in heat exchangers that could allow carbon-monoxide into the air stream blowing through it. You could have a bad pressure switch or a bad hose to it. A pin hole or crack in the hose could cause your problem. A blocked intake or flue pipe or cracked heat exchanger . Check your hoses and pipes, if no problem there, I'd have it inspected by a professional.

I was having same problem about a month ago. I thought it was the pressure switch bought new one worked for about 2-3 weeks then it wouldn't heat up the his to start flame. Exhaust fan just runs. It spins freely and turns. So I don't think that's clogged. I really think it's the condensate drain lines. How does the exchanger thingy crack.

Any other help out there?