When your HP LaserJet 4000 is in use, the toner is heated up and fused to the paper to print your text or pictures. During the course of normal operation, the various plastic feeds and rollers inside the printer may become dirty, which occasionally causes vertical lines to appear on your prints. You don't need to buy a new printer to fix the problem. Getting rid of the vertical lines only requires cleaning the inside of the printer or replacing the toner cartridge.
Hold down the LaserJet 4000s power button for three seconds to power off the printer. Unplug the 4000s 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 4000s 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.
The HP 27A and HP 27X cartridges both have the same toner, the 27X just has a larger capacity and prints longer before needing to be replaced. The cartridges are available through HP or at office supply stores. If your HP LaserJet 4000 frequently experiences printing problems besides vertical lines, the issue may be caused by your driver software. Navigate to the driver download page for the LaserJet 4000 and click the link for your computer's operating system. Select the "Download" icon for the most up-to-date printer driver. Open the file after it finishes downloading and follow the update directions on the screen.
Do not touch the black rubber transfer roller positioned underneath the HP LaserJet 4000s toner cartridge. The transfer roller may stop working properly if it is contaminated by oils in your skin. Do not use any ammonia- or alcohol-based substances when cleaning the interior of the HP LaserJet 4000, as these substances will damage the print rollers and toner cartridge.