Lexmark MS639
It is observed that after printing a black-and-white or monochrome document, light and dark horizontal lines appear on the page. Other scenarios observed were;
Reseat the printer's toner cartridge and Imaging unit.
Perform a Power-on-reset (POR).
Print a test page and check if the print quality defect still appears.
Should the issue persist, change the print resolution to 600 dpi before printing document. To change the print resolution, Navigate to:
LEGACY ID: SO8564
Checking the toner and the imaging unit may help solve the issue of seeing vertical dark lines or streaks on your prints.
Are your prints too dark? Try adjusting toner darkness or loading paper from a fresh package.