Make sure you purchase the right ink cartridges for your specific printer and printer region or have your old ones refilled. Cartridges are not compatible between different brands, and often not within the same brand. HP often designs its inkjet hardware to use country-specific consumables. When you unbox a new HP inkjet printer and set it up, you install a set of ink tanks that ship with the printer. As you turn on the device and get ready to use it, an initialization sequence localizes it to the country associated with the ink cartridges you installed.
HP recommends that you use original HP ink cartridges. XL (Extra Large) cartridges cost more but do provide better value in the long run. They will be able to print at least double the pages of a regular cartridge. Our advice is that if you print often, go with XL. However, ink can dry up. So if you are not a frequent printer, go with the regular cartridges.
You can check the ink levels by using the software installed with the HP Deskjet. Ink level warnings and indicators provide estimates for planning purposes only. When you receive a low-ink warning message, consider having a replacement cartridge available to avoid possible printing delays. You do not have to replace the cartridges until the print quality is unacceptable. You’ll usually install both the black and tri-color cartridges in the Deskjet, but you can use the ink-backup mode to operate the HP Deskjet with only one print cartridge. The ink-backup mode is initiated when a print cartridge is removed from the print cartridge carriage. This printing mode lets you print with a single print cartridge, but it slows the product and affects the quality of printouts.
For your US, European and UK HP Deskjet 3940 Printer you need
HP 21 Black AND HP 22 Tri-color cartridges OR HP 21XL Black AND HP 22XL Tri-color cartridges.
There are also 2-Packs available, like the HP 21 Black and 22 Tri-color Original Ink Cartridges, 2 pack.
Try Amazon's ink selector tool to find compatible ink cartridges for your printer.
Replace the ink cartridges
1. Turn the printer on by pressing the On/Off button (power button), if it is not already turned on. It can take a few seconds for the printer to turn on after you press the Power button. Wait until the product is idle and silent before continuing.
2. Load unused, plain white letter or A4-sized paper in the tray, if you have not already done so.
Remove the empty cartridge
3. Open the printer cover. The carriage moves to the center of the printer. If the carriage does not move to the center, turn off the printer, and then turn it on again. Wait until the carriage is idle and silent before proceeding.
4. Press down on the empty cartridge to release it, and then pull it toward you out its slot. Remove the cartridge from the slot on the left to replace the tri-color cartridge. Remove the cartridge from the slot on the right to replace the black cartridge.
7. Hold the new cartridge by its sides, so that the copper strip is on the bottom and facing toward the printer. Slide the print cartridge firmly into the cradle at a slight upward angle until you feel resistance. Push the cartridge into the cradle. You hear a snap when the print cartridge is correctly positioned. If you are installing the tri-color print cartridge, slide it into the left slot. If you are installing the black print cartridge, slide it into the right slot.
8. Repeat step 4 to 8 to replace the other cartridge, if needed.
Align the new cartridge
10. After installing a print cartridge, align the print cartridges to ensure the best print quality. To align the print cartridges:
– Open the printer Software.
– Open the printer Toolbox.
– Click Align the Print Cartridges.
– Click Align, and then follow the onscreen instructions.
– Recycle or discard the test page.