3 yr. 100,000 btu heil 90 natural gas furnace

If you have the cabinet door off this furnace where the burners, draft motor are located and turn the thermastat up the furnace will go through all of sequences like it's suppose to and will turn off after the thermastat is satisfied and will continued to work like it's suppose to, but the minute you put the cabinet door back on it will go on and through all sequences and run for a few minutes, and the flame goes out and after the blower goes off, it will immediately tne draft motor comes on & it gives you error code of 3 blinks which means the pressure switch is bad, but the minute you take the door off the furnace comes on and continues to run non stop until the thermastat is satisfied. The pvc pipes are run to outside side by side like there suppose to and you can feel the air on the intake being drawlal in. The furnace has worked for 3 years with the door on, what is causing this?