Trane XR 80 furnace wierd problem

I have a 2 year old Trane XR 80 gas furnace that's been trouble free up until I noticed something this morning. When I call for heat. The combustion blower ( not the main blower) comes on as ir should, but after 3 seconds it shuts off, then comes on about 4 seconds later and then shuts off. It repeats this cycle indefinately. So I figured it is one of 2 things. A bad T-stat or some limit switch is bad. When I opened the panel, I noticed a cylindrical limit switch with a clear tube on it which leads into where the combustion chamber is. My guess is that it senses if the combustion blower is on. So I removed the clear tube and blew in it to clear any particles out of it. I then jumped the T-stat terminals at the furnace and the furnace came on with no problems. When I put everything back together, the furnace operated normally. So now I don't know what caused the problem. What usually causes this problem? And also does anyone know what the limit switch with the clear tube on it is for?
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That limit your referring to is a pressure switch. You cleared the moisture from it when you removed it. Not an uncommon occurrence in the spring or fall.
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Thanks, I figured it was a pressure switch of some sort.
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