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Welcome to Just Answer, my name is ***** ***** I will do my best to help you with your issue. If my initial response doesn’t answer your question then let me know and we can continue our conversation.This error is either a draft problem or a clogged condensate drain problem. You need to remove all the hose on the unit an make sure the water is draining freely. Also check the small vac tubing connected to the draft pressure switch ( see attached pic for typical). The tubing should have no water in it and where it connects to the furnace should be open. You can poke a paper clip into that port but not the one on the switch.
Ok, blowing/sucking on the hose to the switch doesn't indicate anything if it clicks. The vacuum that the switch responds to is far lower than what your lungs can create. So since you're sure the tubing and the port the tubing connects to is clear then you either have a bad switch, a weak draft inducer fan or your vent us still partially blocked. The only way to determine which is the likely culprit is to put a manometer on the exhaust and measure the draft pressure. Since you probably don't have a manometer replacing the switch would be the next thing to try.