first of all I've read many posts about black color printing out as gray but I have not yet found a solution that works.
All other colors are printing fine (about 35% remaining), but black has started to turn to gray (50% remaining). I've only printed about 1300 pages.
I have removed the black toner and shaken it. I removed the imaging unit and put it back few times. I did not see any dirt etc inside the printer when the imaging unit was removed.
Try niveks post here.
He doesn't post here any longer (since 2014) but his solution fixes more printers than probably any other post on this forum.
My printers hardware configuration is different than in the example you linked and it does not have any diagnostics mode. I'm hoping some sensor etc is dirty and I just have to find and clean it. Last year printer complained that waste container was full, but one sensor was just dirty.
the printer still has original toners and it is quite old. I tought that maybe the black toner is really empty or gone "bad" some how. I bought new black toner but printing result was excatly same as with the old.