Streaks when printing larger blocks of color

[Snailkite]

I have tried the print head cleaning several times but still get streaks when printing blocks of color (e.g., blue areas have pink streaks in them). I have used HP ink from day 1 (subscription service). Light duty, noticed this problem around 18 months of ownership. Next steps?

[Bob_Headrick]

Blue would be printed with a mixture of Cyan and Magenta (pink).  If you have pink lines in large blocks it would indicate the Cyan cartridge is at fault.  The troubleshooting steps in the document here may help resolve cyan not printing correctly on your Officejet Pro 6978 printer.  Go through the steps in order until the issue is resolved.

[Snailkite]

Thank you for your assistance. I've tried all of the steps and even replaced ink cartridges. Next step is "servicing" but that really means recycling because who would pay for service when you can buy a newer replacement for similar money and less hassle. 2.5 year-old printer.

[Bob_Headrick]

If the standard troubleshooting did not resolve streaking cyan I would suggest removing the cyan 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 the print quality to recover. 

If none of the above restores printing your printer will likely require replacement since you indicate it is no longer under warranty.