HP Designjet T3500 Production eMFP - Manual (Page 217)

First look at the upper part of the print (part 1). Each colored rectangle should be a uniform color without any
horizontal lines across it.
Then look at the bottom part of the print (part 2). For each individual colored pattern, check that most of the
dashes are present.
If you see horizontal lines in part 1 and also missing dashes in part 2 for the same color, you should clean the
printhead, selecting the relevant color group. However, if the rectangles look solid, do not worry about just a
few missing dashes in part 2; these are acceptable because the printer can compensate for a few clogged
nozzles.
Here is an example of a gray printhead in a good state:
ENWW The Image Diagnostics Print 207