I've got an old HP 5si that I recently bought a duplex unit for. However, when I actually try to use it, I get error code 13.8. When I open the back of the printer, I find the page printed on one side, and loosely (and crookedly) dropped halfway down to the turnaround area under Tray 3. Thinking it was the duplexer, I managed to get a replacement one, which was tested before being sent out to me. When it arrived, I installed it and encountered the same problem. Obviously, this doesn't happen when I don't print in duplex mode. I'm not sure whether the page is jamming on its way down, or on its way back up.
What's the possible issue here and how do I fix it (what parts do I need, etc.)? I've been trying to get this brute working properly now for 6 months!