Only the AC is working on a 2007 Ducane Gas Pac. Flashing code says "Pres. Sw. senses

[Mike C]

incorrect pressure"? I have a 2007 Ducane gas pac and the heat won't come on. The code says "pressure switch senses incorrect pressure" The exhaust is blowing fine out of the little "chimney" on the side of the furnace, and the condensate line is clear. I disconnected the clear tube and blew into the pressure switch and I could hear the diaphram (?) moving but the furnace still wouldn't light. Is blowing on the tube going to "test" it? It seems like it should? Is there a way to actually test this switch (I don't have or know how to use a Manometer) or should I go ahead and replace it to at least eliminate it as the cause? The gas stove just on the other side of the wall is working fine. Any help would be greatly appreciated. I'm a carpenter but I'm always impressed by what you HVAC guys can do and can understand. Thanks again, Mike