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Trane TUH2D LP furnace IFC board indicates reverse polarity after 1 sec. into prepurge. Checked grounds and neutrals for stray voltage, none indicated on Fluke meter. Purchase new board, same results. Trane said stray voltage over 2 volts. Installing new board , did not connect ECM2 harness just in case . Again after 1 sec prepure, 6 flashes and cycle stops, then repeats again . How would the dual pressure switches affect in this way? We installed this furnace 16 months ago, no problems last year. Please suggest.