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We wouldn't recommend you to fix the problem yourself because this is a highly unlikely situation here.
Your furnace worked for so many years and of the sudden you got a fault code of reversed polarity?
Have you or electrician done electrical work around your house? Maybe somewhere along the line, a cut off switch to the furnace was messed with and didn't know.
You need a trained HVAC technician to find out the problem for you, too many things have to take in consideration.
Besides! Live 120 volts can (and will) kill you under the right circumstances.