I have a m451n color with duplex. Machine will run 1 page just fine. Do multi first page comes out 3/4 ways and jams and second page is a the registration roller assy. Machine says jam open back cover. replaced feed roller and fuser assy. tried with bypass and same problems. I know it not a firmware up date. And it has only 25k on machine. any thought would be a big help.
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