ive got a thin black line on the front and back of each printout.
what i dont understand is that its on the front and back, even then its a simplex print.
i tried replacing the fuser and transfer unit already without any luck.

Does the thin line appear vertical or horizontal on portrait ?
If vertical on portrait does the line only appear once pr. page on both sides ?
also ...
Verify the printer is clean inside, inspect for damaged rollers that could cause physical marks/lines on the paper.
Did you try with other media in the printer ?
1) Try to put the printer into diagnostic mode selecting "disable cartridge check".
2) Swap positions of the black and magenta toner cartrdige.
3) Try to print a few internal pages and verify if the streak is still black or has changed to magenta.
Please report back with your findings.