Hello you will be wasting your money replacing the switch. They very rarely go that far out of calibration. The only way to know for sure is to measure the vent pressure with a gauge. However what I would do it check the vent for blockage take the draft motor out and make sure nothing got in there. You could have a blocked flue or bad vent motor or restriction in the heat exchanger. The blocked flue or something in the vent motor will be your most common causes. .
Let me know ,Richard
Ok I appreciate the answer although I am a novice, just to clarify, clear the flue/vent, make sure there are no restrictions.Terminology: is draft motor and vent motor what they call the combustion blower motor in the flue?What exactly is the heat exchanger, and is that in the flue or in the combustion chamber?