The control panel buttons like to fail on the HP 4000 printer. In most cases the customer doesn't say anything but when you go to print off a config page you soon realize the issue. It's a pretty easy fix; just remove the control panel and disassemble it down to the button contact pad.

The contacts have a silver coating over them. Most likely it was originally designed to prevent rusting but overtime it prevented the contacts from making proper contact with the board. Take a screwdriver or spring hook and slightly scratch away at the coating exposing the metal beneath, reassemble, and put the control panel back in place.