I found this on https://forums.tomshardware.com and it worked for me on a Pixma mx715 BRIEFLY!
1. Turn OFF Power
2. Open the print head bay (as though you were about to change inks)
3. Turn ON power
4. Wait for print carriage to start moving to the left and let it go past half way
5. Before print carriage reaches left hand side (but after going halfway across) shut the cover.
6. Leave the Printer turned on
7. Good to go.
(Very occasionally I have had to do this twice in a row to get past the error)
Unfortunately before I was able to do anything useful the printer ceased to start up properly. A power monitor showed a brief consumption of 6W at plug-in, then it fell to zero and the power light did not light. (So I am in iFixIt to find a solution to that. The removable power supply has been cut open…)
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