I recently bought some 902XL black ink cartridges for our HP Officejet Pro 6978 printer. After putting a new one in, the printer would not print in black – all colors worked fine. I ran the “Clean Printhead” and “Align Printhead” commands, no luck. I checked to see that the “vent” on the top of the black cartridge was uncovered (it was). I then starting doing general searching on HP forums and YouTube. I found a video that talked about using wet paper towels to clean up dried/excess ink on the bottom of the print carriage. I did that, then ran the “Clean Printhead” command twice, and was FINALLY able to get black ink text on the test page – whew!