Press the cover release and open the printer’s top cover fully.
Note the positions of the four cartridges.
Cyan cartridge
Magenta cartridge
Yellow cartridge
Black cartridge
Make sure the toner cartridge in question is correctly position and the right hand clip is pushed back.
Holding it by its top centre, lift the image drum, complete with its toner cartridge, up and out of the printer.
On the right hand side of the toner look for a sensor patch, make sure this is not dirty. This patch may be different depending how old your consumable is. If the patch is dirty it can be cleaned with a cotton wool or soft object. Now look inside the printer opposite to where the sensor patch would sit. There is a black square with a sensor located inside. Also check to make sure this is not dirty.