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The printheads in the 6800 series are not user replaceable, and they are prone to failure. It seems Hp only intended those machines to last for a year, two years max.
What you can try is removing the ink tanks, all of them. Then, clean the electrical contacts on the ink tanks. And also clean the electrical contacts on the printhead itself. If that doesnt work. Id read over this HP forum so you can get an idea of the issue and how widespread it is with the 6800 series.
If the printer is under a year old, HP will replace it for free. if its over a year old, you could try calling HP support but almost certainly they will say its past the warranty and youre out of luck.
What I suggest, if you cant get the printhead working is to replace the printer with a model that has a user replaceable printhead.. That way if the printhead fails you can actually remove it completely and try to clean or repair it, otherwise you can replace the printhead instead of the entire printer.
The MX 922 by canon is a decent machine, with comparable features to the HP you have. It also has a user replaceable printhead and generally speaking, should last a bit longer then your hp did. http://amzn.to/28Vh5ID
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