Since the issue persists despite your thorough troubleshooting efforts, it's likely that there's an internal hardware issue causing the error code C000 on your Canon Pixma MG6620 printer. However, before considering professional assistance, there's one more step you can try:
Perform a "Factory Reset" on your printer. This step can sometimes resolve software-related issues that might be contributing to the error code. Here's how you can do it:
# Turn off the printer and unplug it from the power source.
# Wait for about 5 minutes to ensure all residual power is drained from the printer.
# Plug the printer back in and turn it on while holding down the "Stop" button (the button with the triangle inside a circle) until the Alarm lamp flashes.
Once you've performed the factory reset, monitor the printer to see if the error code persists. If the issue continues, it's best to contact Canon support or a professional technician for further assistance, as it's likely a hardware issue that may require repair or replacement.
I hope this solution helps resolve the error code C000 on your printer, and you're able to get back to printing smoothly! If you have any further questions or need additional assistance, feel free to ask.