Called Van's. My furnace was only blowing cold air - no heat. LED indicator said "Limit control switch" problem. Van's guy came and said it was a bad circuit board. Needed to be replaced. $800 ($600 with club membership). Asked if it could be limit switch. Told "No."
Called .... for second opinion. Came out. Said "Bad limit switch". Replaced it. Ten minutes later furnace was working again.
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