I am working on an HP4000. Here is what it does. It picks up the first sheet of paper and feeds it into the fuser partially, then it picks up the second piece of paper and feeds it partially into the registration assembly. ( If you pull the paper tray out there is about a quarter to a third of the page hanging down ). Then is shuts down with a 13.1 error.
It will print fine if you print from the Multi Purpose tray.
I have done the following.
replaced paper feed assembly
removed the lower part of the printer containing the paper tray and tried one from another printer.
Tested both PS102 and PS103, both are working.
Replaced the main motor on the right side of the printer.
Replaced tray one assembly, just for grins.
Replaced the fuser.
There is only one main item left that I haven't changed. Don't want to change it if there is something else I can try.