printer not giving clear copies

Hello

I have a HP Laserjet Pro M1536 all in one printer/copier.

Both copies and printed copies are not printing clearly.  Shadowy printed lines are showing underneath clear printed lines and paragraphs.

I have changed the cartridge and also cleaned all rollers etc and still have the problem.

Also have run through the service test sheet, but still the problem persists.

Do I need a new part - the machine is about 4 years old.

Your earliest response would be appreciated.

Thanks

Louise Hedley