My Trane XR80 furnace is only blowing cold air. The diagnostic light is bllinking 6 times which, I think, means the 115 volt polarity is reversed. Is this something I can fix or do I need a repairman?

Which, I think, means the 115 volt polarity is reversed.

Did you get the above info off the furnace legend?

If the polarity is not reversed it is probably a bad circuit board.