Hi maineman42,
To reseat the printhead in your PIXMA MG6821, please follow the steps in the following Knowledgebase Article:
https://support.usa.canon.com/kb/index?page=content&id=ART143852
If the 1403 error persists, 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, or for some reason are incompatible with a new or upgraded system. This option allows you a one-time opportunity to purchase a replacement product, discounted from the list price.
To help you get up and running quickly, we also offer free standard shipping (if the order is completed by 12:30PM ET). If you would like to take part in this option, please call our Sales Department at (866) 443-8002 seven days a week, 8am to Midnight. Let them know you have been working with technical support and the Canon Upgrade Program was offered.
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