Canon MP640 doesn't recognize paper

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All of a sudden, my two-month old Canon Pixma MP640 is giving an 'out of paper' message for plain paper from the tray. There were about 10 pieces still in the tray at the time. I filled the tray and rebooted the printer, but no change. The printer clicks a few times like it's looking for paper, then gives the error message. A few minutes before this happened, I had replaced three cartridges and printed a one page duplex job successfully. No paper jam that I can see, and all settings look right. Any ideas? Thanks,

Update: After posting, I noticed the lengthwise paper guide was set back at A4, tho I think it had been there all along and, in any case, some of the paper was pushed forward. Readjusting the guide to the 8.5 mark got the paper rolling correctly.

 

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Further update: this (next) morning it didn't work again. Called Canon and they are sending a refurb replacement.