Unable to print black

[Winj]

Printer was fine until yesterday.  I cleaned the print heads (via computer internal program) 3 times.  It prints color, but no black.  I have just replaced black cartridge, but it still won't print black.

[Bob_Headrick]

Please read this post then provide some details.  What printer model? What operating system?  Are there any error messages on the printer or computer screen?

[Winj]

HP Office Jet Pro 6978,  Windows 10

[Bob_Headrick]

The troubleshooting steps in the document here may help resolve black not printing on your Officejet Pro 6978.  

If the standard troubleshooting did not resolve black not printing I would suggest removing the black cartridge.  Turn it over and examine the area around the ink port.  There are four orange plastic spots, left over from the protective cap.  Make sure these are flush with the cartridge body, if not carefully trim them flush.  If the plastic nubs protrude they can prevent the cartridge from seating properly.  Before replacing the cartridge in the printer dribble a few (maybe four) drops of hot water into the ink port in the printer and two drops into the ink port on the cartridge then snap the cartridge back in position.  

It may also help to manually clean the printhead as described in the post here.

I have also seen cases where leaving the printer overnight after the cleaning cycles allows black to recover.

If none of the above restores black printing your printer will likely require repair or replacement.