I have a guardian upflow furnace, 92% eff. It is sending me a #3 error (pressure switch is stuck open), it had happened to me about 3 weeks ago, turned off the furnace for about 15min and the error cleared and it has worked fine ever since, now I am getting the same error but cannot get it to clear and am not sure what to do to get the furnace to start again and hopefully stop this from happening in the future. Thanks for your help.

Looks like you will be needing to replace the pressure switch unless you have the capability and tools to check the system further.