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Sounds like you need to clean the printer components.
Give me a few moments to pull the technical manual, and tell you specifically what you need to look at and clean.
YOu said you did a diagnostic page......yes?
What was the results?
Did you SHAKE (yes, literally shake it, like a polaroid picture) the NEW ink cartridges about 6 times before you installed them in the printer?
Theres quite a few different things that can cause the prints to come out not as dark.
The ink tubes can be pinched or kinked?
Did you use Genuine HP ink cartridges?
I also want you to check the print quality settings.....and make sure they are as they should be.
See image link below for instructions on checking the print quality settings.