If you have a paper jam on a on a HP OfficeJet Pro X451DN, you will want to check in the duplexer unit as often, that is where it will hide.
Fixing the Jam
01. Turn off the HP OfficeJet Pro X451DN using the power button.
If the power button will not work, pull out the power cord.
02. Leave it unplugged for 60 seconds to cool.
03. Open the left side of the HP OfficeJet Pro X451DN.
04. Remove the duplex module at the bottom
05. Next, lower the blue lever in the middle.
06. Look for any pieces of paper.
07. Put everything back
08. Power up.
09. Try to print a report directly from the printer.
If there are curled up paper hidden inside, this will force it out.
09. If it jams again, leave it on and follow steps 3 to 7.
You should now see the paper and can remove it. Once closed, your report it print.
Warning: When removing the duplex module avoid making direct contact with the black cylinder to prevent ink smear (on skin, clothes, etc) and keep it level to avoid spilling any maintenance ink. Also, when removing the duplex module, do not let the bottom of the duplex module touch or rest on the ribs on the left door as it can damage them and might lead to paper damage and jams.