When printing diagnostics page, all colors print good. Black doesn’t print a block at all.

[Dj800trouble]

When printing a diagnostics page to see if all colors are printing, all colors print the boxes as they should but, the black doesn’t print a box at all. 

I can print out a diagram in what seems to be black ink. The lines are all very black looking. I can print a 8x11 document that seems to be in only black. The person who wants to buy the Plotter, asked to see the diagnostics page print all colors and black. What to do? 

[Valsimot]

It looks that you need to replace black printhead for start. The printheads ( 4 pcs, one for each color) are customer replaceable parts and they need to be replaced when print quality gets low.

Do you know how old your current printheads are?

https://support.hp.com/lt-en/document/bpp03062

It's nothing to worry about it's totally normal to check the diagnostic page to check the quality of the printheads especially if you are buying a used Designjet because purchase on 4 new printheads will cost money.