Hi Mercer29,
To resolve this issue, please follow all of the steps provided on this page from the PIXMA MG6821's manual:
MG6800 series > Maintenance > If Printing Is Faint or Uneven, Missing Colors
If yellow still does not print after follow the procedure provided in the manual, your PIXMA MG6821 would require service. You are eligible for Canon's Upgrade Program. This is for out of warranty units which have been diagnosed to require service. This option allows you a one-time opportunity to purchase a replacement product, discounted from the list price.
If you would like to take part in this option, please call our Sales Department at (866) 443-8002 Monday through Saturday, 9am to 9pm. Let them know you have been working with technical support and the Canon Upgrade Program was offered.
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