All venting, vacuum, and drainage lines were checked and are clear. They believe that the cold snaps we've had are to blame for the vacuum switch(s) being affected since they are so sensitive.
If cold weather hinders pressure switches, they certainly don't belong on a furnace. It's pretty rare I find an actual bad switch. Usually it's a motor problem or blocked vent. But I am glad to see you and they have covered all basis before replacing parts.
Noticing that you had other problems prior to this event, I would also look very closely at the heat exchanger. A hole in the heat exchanger would create this fault.
It's pretty rare I find an actual bad switch. Usually it's a motor problem or blocked vent. But I am glad to see you and they have covered all basis before replacing parts.