There are several sensors linked with the ink cartridge tray. For each cartridge, the following sensors exist: ink cartridge detection sensor (marked red), remaining ink sensor (ink level sensor, marked blue) and some chip contacts (marked green, to read the ink chip).

To check the first two sensors, enter the maintenance mode. For reference on how to do this, see here. (General navigation in maintenance mode is also described there.)

Press the keys 3 and 2 to enter maintenance mode 32 (sensor operational check). The annoying beeping can be deactivated by pressing the SET key (navigate through the keyboard by using the << and >> keys).

Press the Mono (or Mono Start) key once. If no cartridge is installed, the top line in the display should be empty.

Test the ink detection & remaining ink sensors one after another by inserting a thin opaque object (e.g. a sheet of paper) into the sensor slots. If the sensor is intact, two letters should appear on the display for each sensor (first letter = sensor type (E = remaining ink, I = detection sensor), second letter for ink color, i.e. cartridge slot)

If any sensor is defective, the corresponding letters do not show up on the screen when the sensor is blocked. If all cartridges are installed and contain enough ink, the display should show all symbols, i.e. IKIYICIMEKEYECEM.

Exit sensor operational check by pressing the Stop button. Maintenance mode can then be exited entering 99.

If the sensors are not defective, re-insert all cartridges and continue with the next step.