This started recently. When printing a Word or Excel document using black only, I get small red and blue dots all over the page. But when I print using a color, there are no dots. The dots appear to be like ink was splattered on the page, but it's not really random. There's a definite pattern. I uninstalled and re-installed office. This is the same on my Windows PC and my wife's Mac, using the same printer. Any ideas?

It sounds like a faulty or dirty printer. See if you can borrow another to see if the problem remains.

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Graham Mayor - MS MVP (Word) (2002-2019)
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I agree with Graham but you state "When printing a Word or Excel document using black only".

Why have you only mentioned "Word or Excel"? Does it happen if you print a pdf document or something from the web "using black only"?

Printer model and how old is it? If it has been used a lot it could be at the end of its service life and need a new maintenance box installing but in this case I would have expected you to have seen warning messages pop up.

This strongly suggests that it's a printer issue.

Are the colored dots always in the same place? You might try printing the document to a PDF, then printing the PDF.

They’re pretty much in the same places. PDFs print clean as do emails. It’s only Office in black.

You said you tried reinstalling Office; did you try reinstalling the printer? Or checking for an update? Or trying al alternative driver?

Having the problem on both Mac and PC really suggests that the problem is on the printer side of things.

Sounds like an 'Office 2021' bug that will get fixed at some time.

As Graham said try another printer to confirm. If it happens on that as well it suggests a bug with Office 2021.