Hi Kaitaz,
Thank you for using the Support Forum. Please try cleaning the toner density sensor by following the directions in this article from the online support for your printer. Please consider contacting your support centre for further assistance.
I replaced the full set of drum/imaging units in August. Now my printer won't print in black. I have changed the toner but this has made no difference. It's too expensive to purchase another new set of units just to check the black one. Anyone have any solutions