Model MPGA100B4B Ducane shows pressure switch stuck open/closed but have checked and replaced it. Old one was fine. No blockage in tube or port. Changed circuit board and ignitor. Still shows code for pressure switch stuck. Any help would be great

Make sure your wiring is correct & in tact. Check the exhaust pipe & intake, if so equipped, for blockage. A quick test is to disconnect the exhaust to see if the furnace will light. If it does, the problem is in the exhaust pipe. It must slope upward away from the furnace at a slope of 1/4"/foot of run with no bellies or sags.
Sometimes a plugged condensate drain or trap will cause that code as well.