Officejet Pro 8500 A909g Wireless suddenly won't print yellow; no error messages

Question

Is it possible that a printhead needs to be replaced even when Diagnostics Pages all say "Printhead health:  Good"?   I don't have any error messages.  There are no obvious white lines through the colored blocks on test pages.  In a couple of blocks on some test pages, there is one, very fine, barely visible line, but it's not the same on all pages.  (Today I've printed about a dozen test pages of three different kinds that have colored blocks on them, and there are no white lines like the ones shown in examples of bad test results.)   I have used the tool for cleaning the printheads many, many times today and have cleaned at all of the levels.  The ink level in the yellow cartridge is fine, and it's been working well until yesterday and today. 
The printing of the five colored blocks on test pages is good, bright, and clear.  The only problem is that the block which is supposed to be a light orange color is only pink, and the block that is supposed to be green is only blue:  they are lacking yellow ink.
I haven't had any problems with the paper I've bought at Staples for six years (I also bought my printer at Staples ).  The printer is six years old but has not been used hard.  I use it regularly for a page or two a few times a week, so the printer isn't sitting idle for weeks at a time. 
I read something in "help" information about using a straight pin to gently remove excess adhesive from the nozzle of a new ink cartridge, which didn't apply to my cartridge, but I decided to experiment anyway by pushing a pin through the nozzle of the partially-used, yellow ink cartridge,  The test page I printed after that is the page with just four colored blocks on it rather than the five blocks on a different test page.  This is the only time in two days that the yellow ink has printed.  However, it printed in a wide horizontal band a little streaked with gray in the middle of the block.  Then there are fairly wide white bands (no ink) both above and below the wide yellow/gray streaked band in the middle, and there are very narrow, yellow, horizontal strips  printed along the very top edge and the very bottom edge of the block.  In other words, the sequence of horizontal printing in this block only is:  narrow yellow strip, wide white band, very wide yellow/gray streaked band, wide white band, and narrow yellow strip.
It seems this might mean that some yellow ink actually got through after I'd poked the pin into the center of the nozzle of the partially-used ink cartridge?  However, when I tried printing the same test page again, there was no color, no ink showing at all where the partially-printed, yellow block had been on the previous test page.  There was only a totally blank, white space where the yellow test block was supposed to be.  The blue block, the red/pink block, and the black/gray block all printed well.
When nothing solved the problem of yellow ink not printing, I replaced the partially-used, yellow cartridge with a new yellow cartridge, but that didn't make a difference either:  the yellow ink is still not printing.  Yes, I use genuine HP ink cartridges.  I buy new ones whenever I need them -- never refill them or use any other brand but HP. 
I used the alignment tool once to fix some slight wavering in the lines on a test page.  The lines are now perfectly straight.
I apologize for being so wordy, but I wanted to send all of the details that might help diagnose the problem.  I read other questions and answers in the forum about yellow ink not printing, but while there were some similiarities, none of them was 100% exactly like my experience with the problem.  What shall I try now?  Thank you for any suggestions.

Answer 1

Good day.
Can try the steps in this document?
http://h20565.www2.hp.com/portal/site/hpsc/template.PAGE/public/kb/docDisplay?javax.portlet.begCache...
Although I am an HP employee, I am speaking for myself and not for HP.
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