Hi,
Try this:
1-Remove the duplex unit and lower the plate there and look for any paper jam
2- Clean the paper sensor, open the left cover, there is a clear plastic square on the middle. use a lint free cloth damp with water
3- check this video, http://link.brightcove.com/services/player/bcpid1190528646001?bctid=REFIDBC2621062496001
I hope this helps
Best regards
Mike G