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?

Sounds like there is an obstruction in the combustion air pipe. Even if you can hear air being pulled in, it may not be enough. Disconect the intake air pipe and see if furnace works with cover on. If it does, there is definitely a small obstruction in the pipe. Have to cut it apart to find what's in there, most likely a bird, but could be anything. If the furnace is not getting enough combustion air, it will cause the combustion air pressure switch to not close, giving you your three blink code.