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Only a black and white wire that runs from the thermastat to furnace

Sounds like the blower relay is stuck in the on position from your description. If your furnace has a circuit board this relay is usually located on the board and is not a service replaceable part....

I have a 2001 Tempstar DC90 Ultra High Efficincy furnace ! The problem is that the fan will not shut off when the desired temp. gets reached?? I have it set for 70 degrees and it lights up heats up...