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You may need to remove the print head and clean it
To do that you will need to remove all of the ink cartridges, and use a clean, damp, lint-free cloth to clean off any ink on the nozzle areas.
If you are finding that the water isn't cutting it, you can use isopropyl alcohol.
The manufacturer recommended instructions for the cleaning (complete with diagrams) can be found here: http://h10025.www1.hp.com/ewfrf/wc/document?cc=us&lc=en&docname=c01661383
While you have the printhead out, look at the area where the ink tank connects and make sure there isn't any dried ink there. If there is, you will need to clean that off as well. If you still find the printer won't print, there may be dried ink in the passageways to the nozzles. In that case, soak the print head in warm water. Sometimes syringes can be used to help remove ink from the passages using pressure or vacuum. A 5cc syringe (without needle) which has had it's nozzle widened can be used to draw cleaning solution or hot water into the passages via vacuum.
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