E525w blue lines

i recently got a E525w printer as a gift. It's more than 5 years old, hence no guarantee or alike.

I read a bunch of similiar posts in a subreddit, as well as on dell.com and other pages, but none of them provide a working solution.

I tried everything the user manual suggests, including

• - updating firmware & driver

• - reset / calibrate / adjust all the things one could via the printers menu

• clean the "CTD sensors" in the back

• re-insert the catridge

but my prints still get cyan/blue coloring, eventhoug they should be only black.

(In another post a reddit user revealed that this E525w model is comparable to two xerox printers but neither the 708P90826 service manual gives me a hint, what part i should clean / replace / fix.)

Any ideas?