Canon NP6551 Operator's Manual page 188

Remove any paper remaining in the duplexing
10
unit.
Push down the left cover as shown in the figure,
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and remove any jammed paper.
Push the duplexing unit back into the main unit.
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When the unit is pushed in as far as it will go, a
tone will sound.
Open the duplexing transport unit and remove
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any jammed paper in it.
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