This is the best Linux colour laser printer ever made. It can use the cheapest fake toners, and all the scrapings are in the toner cartridge. But mine pooped out the other day with 'CTD scanner error'.
I read everything, and ordered a new fuser. That's because all the ink was smudging when you touched it. New fuser went in, but the same error message.
The sensor is on the bottom. You can get lots of instructions, but I couldn't see it. I just wiped everything nearby with a toilet wipe. I got a bit of black toner, and then everything worked.
The ink stays on the page now.