Jam related to duplexer unit

[Meditator]

Dear Members, I have a "jam" problem with an HP4700dn color laser printer.     I have TWO of these printers.    When I swap the duplexer unit, the duplexing problem stays with the printer rather than moving with the duplexer unit, so I assume that both my external duplexers are functioning as they should.     The bad unit appears to perform the first half of the duplexing function, but then discharges the sheet loosely into the front cavity of the printer.   The annunciator panel indicates "13.12.00" as the type of jam.   But it isn't exactly a jam, because the paper is loose.  Nevertheless, I have to open and close all the printer doors to clear the error message.   The problem happens every time I try to print duplex.   There is no problem with simplex printing.    Any thoughts on a solution would be most welcome.    With thanks.

[Repairatrooper]

Been a while since I worked on this model but 2 most common causes are a sagged exit flag or a failing pickup assembly.

[Meditator]

Thank you REPAIRATROOPER for taking the time to respond to my inquiry.    I have discovered the cause of the problem, as follows:   Prior to getting in touch with the forum, I attempted to upgrade the HP4700n to an HP4700dn by adding a new duplexer unit to the top of the machine.  One would think that the duplexer unit alone would enable the printer to duplex, but it didn't.  It repeatedly indicated a "jam."  It turns out that there is *another* mechanical part to support the duplex function INSIDE the printer.  This is present in the native HP4700dn, but NOT present in the HP4700n.    The interior part is called the "duplexing feed unit" and features "oblique rollers."   If this is missing, the first side of the sheet will print, but then the sheet will just drop into the cavity at the front of the printer because the secondary feed unit is not present to receive and carry it.   The duplexer that attaches to the top is available from several vendors on eBay, but I don't know if the interior feed unit is available anywhere.  Since the machine is now 14 years old, the inquiry increases in its obscurity.    Thank you anyway!