The Quick Answer
If your HP OfficeJet Pro 6978 won't print black ink, the black printhead nozzle is likely clogged with dried ink. To fix it, swipe down on the printer's control panel, select the Setup (gear) icon, go to Printer Maintenance, and select "Clean Printhead". You may need to run this tool up to three times to fully clear the blockage.
Why Black Ink Stops Printing
The HP 6978 uses liquid ink. When the printer sits idle for several weeks without printing, the microscopic drop of ink resting at the tip of the printhead nozzle dries out and forms a physical crust. The printer still thinks it is spraying ink, which is why it goes through the motions but produces a blank page.
Step 1: Check the Cartridge Vent
Before running a cleaning cycle, ensure the cartridge itself isn't the problem.
- Remove the black ink cartridge (HP 902).
- Inspect the top of the cartridge. Ensure the orange pull-tab was completely removed during installation. If plastic film is still covering the tiny vent hole, ink cannot flow out.
- Use a pin to gently clear the vent hole if it appears blocked by adhesive.
Step 2: Run the Printhead Cleaning Tool
The automated cleaning tool forces a heavy surge of fresh ink through the nozzles to dissolve the dried crust.
- Load plain white paper into the input tray.
- On the printer's touchscreen, swipe down from the top to open the Dashboard.
- Touch the Setup (gear) icon.
- Scroll down and select Printer Maintenance.
- Select Clean Printhead.
The printer will print a Print Quality Diagnostic Page. If the black text is still missing or faded, follow the on-screen prompt to proceed to the Level 2 and Level 3 cleanings.
