If you mean the larger vent holes on the top of the unit, yes, keep those plugged. But the smaller ones, (located on top and bottom of an LG) keep those open! You must have fresh air entering the unit. This is irregardless of whether you have a fan or not. All that fan is doing is keeping what air is inside the incubator moving. (i.e. circulation) It does not draw in fresh air. If you plug all of the small air hole vents, then you run the very real risk of suffocating your chicks. Especially if you have a large number of eggs hatching all at once. The smaller air vent holes will not normally affect your humidity nor temperature levels to any appreciable degree, so just leave them open. Good luck, and let us know how your hatch goes!
. In that case, I'd say you've pretty much got it covered! Go ahead and plug the larger vent holes. If the humidity gets too high and you think you might be running the risk of drowning them in their shells, you can always pop one more vent open.
You have an O2 sensor? Where did you find one of those? How much did it cost and does it take up much room in the incubator? I'd really like to see that! It was something I was constantly worried about once all of mine seemed to be popping out of their shells all at once!
Aw nuts! I was afraid you were gonna say something like that. Those units have a tendency to be very pricey....enough to be out of the range of my pocket book at any rate. I'd still love to see a picture of it. Ya never know what crazy things you can find at an auction from time to time!