Hi there. The Printer Doctor is in and ready to help you! Please answer all my questions about your problem so that I can give my best diagnosis.
What an awful sight for you! Okay, it sounds like you are having what's called "cross-contamination" of colors in the printhead. That means that somehow ink flowing from the ink tanks into the printhead have mixed together. This can happen if the printhead has become faulty due to age and is wearing out.
However, it could be a defective cartridge as well. Try composing a 1/4 page block of a yellow box in MS Word. Copy and paste it down the length of the page, then print it several times. If the contamination is not too deep and the printhead is still okay this repeated printing may flush out the contamination.
Also, perform the same test on the other colors as well to make certain the they print true to color. If only the yellow cotinues to print incorrect color, try changing out the yellow cartridge only.
If after several cleaning operations and/or changing the cartridge it still does work properly, you will need to replace the printhead altogether. You can get the printhead directly from Canon Parts (Customer service), or by searching on Google for Canon PIXMA MP600 printhead. If you end up needing the printhead and cannot find it, let me know and I will help you locate a resource.
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Thank you for trusting yahoo, and HAPPY PRINTING from the Printer Doctor!
Thanks, will let you know how I get on. Thelma