If you need to find out which cartridge to replace after receiving a "low toner" or "replace toner cartridge", look at the Home screen on the printer, there is an icon that represents a toner cartridge, use the navigation keys on the unit (up, down, right, left arrows), to go to the cartridges area, there the levels can be seen to define which cartridge (s), need to be replaced, or prevent running out of toner.