Have you tried replacing the black imaging unit?
We have a C405 printer that has been printing fine until the previous cartridge. Black text prints very pale, however any black in images appears normal. This happened on the previous cartridge, we replaced it, and it's still happening on the new cartridge. I tried cleaning the fuser unit but this didn't help. I am unsure what could cause this as the printing is fine on an image. I have also tried colour calibration, but when scanning the calibration page an error occours every time.
Would greatly appreciate any help!