Hi SandiC,
Please unplug the power cord from your PIXMA TR7820 for about 30 minutes. After that amount of time, reconnect the power cord and turn your PIXMA TR7820 back on. If the 5C03 error appears, your PIXMA TR7820 would require service.
If your PIXMA TR7820 was purchased less than a year ago, please contact one of Canon's Technical Support Representatives via phone or chat to obtain your in warranty options. To contact a Technical Support Representative, please use the link below to log into your My Canon account:
https://mycanon.usa.canon.com
Once logged in, click on your PIXMA TR7820 and then click on the Product Support button. When that page loads, click on either the Phone Support button or the Chat Support button to access support.
If your PIXMA TR7820 is out of warranty, 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.
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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