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Hello, I have a Carrier furnace that is suddenly blowing the 3 amp fuse. I finally isolated that the COM terminal is blowing the fuse. If I connect the blue t-stat wire to it, the fuse will blow. I...

The blue wire is purely to power your thermostat. It is a common wire that connects to the Com terminal in your furnace. You can simply buy a t-stat that is battery operated and ignore the blue wire.

Is your thermostat switched to heat? Is your gas on? It's possible when you pulled your theromstat off the wall, you disconnected a wire on the back. That's all it would take