How do I clean toner dump

The toner dump for this printer is built into the cartridges. They clean excess as they go. Therefore cleaning it would be a part of rebuilding the cartridge or refilling the cartridge with toner properly. The printer is setup to clean the paper path, with a clean page, every time the back toner cartridge is replaced, every 5000 pages. The in depth 334 page user guide is here. If you are experiencing poor printing quality that leads you to believe the toner dump is full or needs cleaned there are instructions on page 209 of the user guide describing how to manually enter cleaning the paper path both from the HP toolbox and the user control panel.