Brother MFC-8420 User Manual page 371

3
Put the duplex tray and paper tray back into the MFC.
■
The Jam Duplex message appears if you remove a sheet
of paper from the Face Down Output Tray after only one side
has printed.
■
If the Paper Adjustment Lever for Duplex Printing is not set
correctly for the paper size, paper jams may occur and the
printing will be out of position on the page.
22 - 11 TROUBLESHOOTING AND ROUTINE MAINTENANCE