Actually...the nipple that the vacuum line goes to doesn't have an associated switch; the line goes from the gas switch to the side of the combustion chamber - the inside of this nipple simply opens up into the chamber.
My guess (and it is simply THAT....a guess) is that this vacuum plays a part in the whole "sealed combustion" system, and is simply a safeguard of some type. I checked the hose and nipple, and they are clear (no obstructions).
Regarding your comment of the flame sensor rod.....I'm having a hard time believing that the unit is shutting off due to the rod not detecting a flame; for the most part, once the gas switch clicks (engages, switches, etc....
), the system shuts off immediately....within 1/2 second. At those times where the unit actually does fire up, it takes a good second to 1 1/2 seconds for the flame to completely light. My perception is that the system is shutting off much sooner than it would if it were actually waiting to sense flame.
Someone I know had suggested that I try jumpering the pressure switch, just in case there is something wacky going on during the whole process. I'm going to try that in a little while, just as soon as I'm done replacing the handle on my kitchen oven (I'll jumper it once the inducer fan starts blowing) just to make sure it isn't working marginally.
Man...for a computer networking guy, I'm sure getting a crash course in the operation of a furnace lately!!!