Each printhead contains 21 vertical pins and prints two lines at a time. A dot missing from a portion of every other line designates either a failed printhead or driver electronics.

Multiple rows of dots missing could be due to loose cabling.

1. Inspect the printhead cabling and printhead driver board cabling.
   
   
2. Exchange the cabling between two printheads, or change printhead positioning.

   If the original (or same) printhead continues to fail replace it (and re-cable properly).

   If the other printhead now fails, replace the associated printhead driver board (or the associated printhead cable if failure continues).

Randomly missing dots may be caused by too wide of a platen adjustment. Also check for improper ribbon travel through the ribbon guides.