The first place to start is the input tray that is being used to print from. Remove the paper and check for any misloaded pages, bent corners folded under causing the stack to be uneven will most likely cause a skewing or false jam as it cannot pick the paper out of the tray. Next check the paper pickup/feed rollers, they last quite a while and can be swapped from another tray for testing purposes. The 9000 supports so many different sizes of printing media. The tray guides can adjust to print almost any size in the custom setting. So having these guides set to the exact size is important, even when printing standard sizes of media.
13.12 Jam in duplexer error, opening the left door you find a jam in the duplexer, slightly skewed. With further testing, using the built in diagnostics "paper path test" you find it only jams from tray 4. Trays 2 and 3 work just fine so now you know the duplexer is not the problem also. Checking the tray 4 guides it appears they are set for the correct size. Remove the paper from tray 4 and move the front to back guides several times. This will reset the guides to the correct position and prevent the paper from skewing and jamming from tray 4.
Another very common error is the 13.20 jam indication. Either a false jam ( no paper in the printer), or a real jam with pages throughout the printer. Printing an error log will be helpful in diagnosing what the cause might be. The event log might show some other jam indications that will be useful in troubleshooting the printer. The HVPS contacts on the HP 9000, labeled TB1009 and TB1010 can give false 13.20 jam errors. These contacts monitor the pressure roller bias connections between the fuser and the HVPS. These connections are routed through the fuser connectors and complete the fuser wrapping detection circuit. The wrapping jam detection circuit is an arm on a solenoid that contacts the fuser pressure roller when this check is performed. The printer will then compare the reading to the required value for any differences, and report back a wrapping jam if the value is out of the operating range. A wrapping jam is when paper wraps around the pressure roller. If this error is displayed with no paper wrapped around the pressure roller then check/ reseat the contacts and springs under the HVPS.
There is another contributor to the false 13.20 jam "front or right door" message. It might appear during or right after the power-up sequence, before any pages have been printed. Of course broken or stuck paper sensor flags can contribute to this error as well. So a visual inspection would also be helpful. Older versions of the DC controller had a very narrow tolerance for the measurement of the conductive coatings on the fuser rollers. This can be why this often comes up after replacing or installing a fuser, leading you to think the fuser is bad!!! There will be natural variances in the conductivity of these rollers which put some fusers outside the operating parameters. The problem was corrected with later versions of the DC controller. Print a configuration page and check the "printer number" ( a two-digit code ranging from 15 to 1C). If the second character is a number (5-9), the printer has an older DC controller which will be more prone to false 13.20 jam errors. The newer or revised DC controllers will have a letter for the second digit, such as A, B or C.