The HP 3900 is a deskjet printer that can print black and white and colour documents. It requires two ink cartridges: the HP 21 black cartridge and the HP 22 tricolour cartridge. When your printer is running low on ink, one or both cartridges may need to be replaced. Make sure you replace the ink with either an HP 21 or HP 22 cartridge. Once the ink cartridge is replaced, the HP 3900 ink status bar will reset.
Press the "Power" button to turn on the printer. Open the printer cover to move the cradle holding the ink cartridge to the centre of the printer.
Push down on the HP 21 black ink cartridge. This will unlock the cartridge so you can pull it out of the cradle.
Pull off the pink tape on the replacement cartridge. Make sure you are holding the cartridge so the copper strip is facing down. Do not touch the copper contacts because it may damage or clog the cartridge.
Slide the HP 21 black ink cartridge into the cradle. Push down to lock it into place.
Close the printer cover. The HP 21 ink counter will reset after a few minutes.