said by WhaleOilBee:
I had a similar problem if you're talking about the switch that detects if pumping air through the combustion chamber results in negative pressure. There is an intake and exhaust pipe leading outside. After two visits from Enbridge that didn't solve the problem, I discovered that on the intake pipe frost had built up to the point that it couldn't inhale. The exhaust pipe created a plume of steam on very cold days and the intake pipe sucked it in. There is a plastic rodent screen on the intake; the frost built up on that. Removing the screen cured the problem. So far no mice have made a home in my furnace.
Don't know if this is your issue, but might be worth a look.
Interesting, I had read something online about the same pipe going outside and being blocked, next time i'll have to go have a look to see if there is frost built up or something similar.
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