it can be a bent, or broken paper flag - so check where they are located.
if detection is done with leds, check that they operate, and not covered with paper debris !
you find them all along the paper path - first at feed in
find all documents and options here :
http://welcome.solutions.brother.com/BSC/public/us/us/en/model_top/monolasermfc/mfc8460n_all.html?reg=us&c=us&lang=en&prod=mfc8460n_allin the faq's there is some good info !