If when installing the printer with HP Easy Start you get a warning that the scanner does not support “Bonjour” and continue you’ll end up with a working printer but no scanner!
In most cases this happens because you have a dual band router advertising 2.4GHz and 5GHz on the same SSID networks and the printer latches onto the 5GHz network. Simply put the HP 7420 scanner does not work on the 5GHz network.
The solution is to disable the 5GHz network or rename its SSID and ensure your printer does not use it, i.e. is connected to the 2.4GHz SSID. Once done uninstall the HP printer software using HP Uninstaller. Reinstall the printer using HP Easy Start. This time you get no scanner warning and end up with both printer and scanner working.