Open ink cartridge tray and remove cartridges by pushing the gray lever.

Check the ink levels by tilting the cartridges. If there is ink left, it should appear in the transparent window of the cartridge.

It is possible that the ink level sensor detects a cartridge as empty although there is some ink left. To continue printing in this case, just put a opaque tape over one side of the transparent window. (The front part of the cartridge can be de- and reattatched at the slit for easier fixation.)

If you do so, be sure to check the ink levels manually regularily to avoid damage to the print head by printing without ink.