After days of battling with a phantom paper jam that would just not go away, I finally fixed it! Hope this will help someone else out there as well. After feeding several sheets of heavy card stock through the machine at once it kept telling me that I had a paper jam 077-900 "open reg cover". Nothing I could do would fix it. I followed the Dell Support paper jam wizard for my printer http://support.dell.com/support/topics/global.aspx/support/dsn/en/tree?c=us&dl=false&l=en&s=gen&treeid=D6C957F1FBBEFB46E030A68F272833B0&doclang=en&cs=
When I got to the part about doing the Digital Output Feed Test (one of the last pages in the tree and the only helpful one in my opinion) the paper always jammed up under the silver roller just before entering the imaging drum.
I gave up, ordered a new printer, and then thought "what the hey I'll give it one more shot"
After an afternoon's worth of dead ends, I decided to really find the sensor that was causing the error. I noticed I still got the error with the imaging drum removed, so (with the imaging drum still removed) I opened the reg cover (largest back door on the printer) and noticed a small black triangular tab stuck inside the printer. It came back out easily, but seemed to be loose. I decided it was time to shut the power off and unplug the thing before exploring any further, but that's just the kind of careful guy I am. :smileyhappy:
The tab (paper jam sensor) was attached to a long black plastic bar inside the body of the printer, and one side had come detached from the side of the printer housing. I pressed it back into place. It took a few tries but it finally snapped back in place. Error cleared and printing fine.
Hope this helps you in your quest to make the best of your 3100cn printer experience. I think once I run out of toner I'm going to buy a Ricoh!