It's really easy to dislodge the ribbon cable from the front sensor board to the motherboard. If you power it up while it's disconnected you will burn out a small inductor on the motherboard. It's a pretty easy fix to solder a jumper in place to replace the inductor.
See http://www.tivocommunity.com/tivo-vb/showthread.php?t=470071
See http://www.tivocommunity.com/tivo-vb/showthread.php?t=470071