Hi. I have had the same problem with the 3480. I have discovered the problem is actually a design flaw. There is a little lever balanced precariously at the rear of the printer where the paper feeds in. If this falls out of place (which it seems it generally does to anyone who tries to change the ink cartridge) the printer will think the cover is open. Its nigh on impossible to put it back yourself and I have just taped up the switch although it now thinks it has a perminant paper jam...
My advise is to ditch the Lexmark and go for another make (other Lexmark models seem to have the same flaw judging by all the forums)
Matt Smyth, December 2007