Error 13-jam paper HP Laser 4+
HP LaserJet 4, 4m, 4 Plus and 4m Plus Series Printers - False Error Message 13 PAPER JAM
After clearing all jammed paper from the HP LaserJet 4, 4m, 4 Plus, or 4m Plus series printer, the display still reads 13 Paper Jam . Even after cycling the power off and then back on, the error remains.
Open the rear door of the printer. There is a paper sensor near the upper right hand corner of the fusing assembly (just below the right edge of the long yellow warning sticker on the fuser) that can cause paper jam error messages even when there is no jammed paper in the printer. Toggle the thin black plastic lever up and down to make sure it is not stuck or sticky. The lever should be in the down position before closing the back of the printer. If the lever is stuck or missing, the printer will generate a false paper jam message.
Hope it will help you !