Canon MB2100 Series Online Manual page 991

1310
Cause
Size of paper may not be compatible with automatic duplex printing.
What to Do
The sizes of media suitable for auto duplex printing are A4 and Letter. Make sure paper of the correct size
is loaded. Press the OK button to eject the paper and restart printing from the front of the next piece of
paper. The reverse side of the ejected sheet is not printed.
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