check the voltage to the furnace AND the grounding. should be 110-120 volts. should have a good ground reading between a copper water pipe and the circuit board metal plate and green ground wire from it if it has one. use a cheater wire and run it from the water line if necessary to do the test. you can also carefully check from the C terminal on the board by the rwyg terminals to the neutral wire with the power on and should not get more than 1 volt reading or let me know and I can look it up. that is the lennox test for poor grounding which can aggravate boards and make them do weird things. sounds like yours is damaged now but I would do the check anyway to make sure the next one duz not get damaged. make sure U are not getting ANY voltage from the W wire to the board in the off cycle. remove the wire and check it to C on the board (press the door switch in with duct tape to do the test) your tstat may be bleeding a partial voltage and aggravating the board.
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