HP 4000 series printer starts printing pages with vertical lines for no reason, display reads "Printing engine test" or "Print engine test"

Ok, easy enough to fix. On the left side of the printer just below the fan vent is a small hole with a print engine reset button in it, using a pencap or some plastic thingie, press that a couple times to see if you can unstick it. I had to actually smack the side of my printer to unstick it. Do this while the printer is turned on, then turn the printer off for a couple minutes and turn it back on. The error should be cleared, if not then repeat as needed.

If that doesn't fix the problem, then the printer is broken :( and probably needs a belt or board replaced.