The black spots on a print image may be improved by the [Fusing Roller Cleaning].
Load a paper (A4 or LT size plain paper) to the by-pass tray.
Perform the [Fusing Roller Cleaning] (toners are consumed).
Menu > Maintenance > Quality Maintenance >Fusing Roller Cleaning
A paper is fed and images are printed on both sides of the paper.
Check that the black spots do not appear any more.
The effectiveness of the fusing cleaning varies depending on the types of images printed or papers used until now. Therefore, the problem may be improved by performing the [Fusing Roller Cleaning] several times, which consumes the toners.