How to Fix Laser Printer Vertical Lines on the HP LaserJet 4000

Hold down the LaserJet 4000's power button for three seconds to power off the printer. Unplug the 4000's power cable from the wall outlet.
Lift open the plastic cover at the top of the HP LaserJet 4000. Grip the front end of the plastic toner cartridge and pull it forward slightly. Lift the cartridge out of the LaserJet 4000.
Use a soft cloth to wipe off the exposed plastic paper feed guide toward the back of the open area inside the HP LaserJet 4000. Lift up the green handle at the far right end of the plastic rectangular paper access plate. The plate is positioned toward the front of the open area.
Wipe off both sides of the paper access plate. Push the green handle back down and slide the toner cartridge back inside the HP LaserJet 4000.
Set the top plastic cover back down and plug in the LaserJet 4000's cable. Turn on the printer and print a text document to see if vertical lines still appear.
Unplug the printer and open the top cover again if the lines persist. Pull out the toner cartridge and insert a new HP 27A or HP 27X toner cartridge. Close the top cover and power on the printer.