Open the printer and remove the grey cover. In the back there is a green printed circuit board. On the back of that the is a small black switch. A white lever is supposed to be attached here, but it always falls off. If you can find the lever, put it back on, or else, find a small piece of plastic or wood or something and duct tape it so that the switch is always in. Problem solved.
For pictures, go here:
http://www.fixya.com/support/t217347-lexmark_x3480_printer
Torsten Jansen, January 2009