blower fan won't shut off
 

Hi
I have a ComfortMaker forced air gas furnace.Model #GUH075A012CIN.Ever Since it was new,it doesn't shut down after it has gone thru its cycle,that is it continues to blow cold air until u manually shut the furnace off,or turn the thermostat down.This doesn't happen all the time though,which is baffling.It has a troubleshooting code on it which says that the pressure switch is stuck open.I have replaced the pressure switch,put it a new thermostat,cleaned sensors,but problem is still there.Could this be a venting or condensation problem

Any suggestions would be appreciated
Tks

Thats what the pressure switch does. It proves venting. Yes the problem could be condensation or a venting issue.