Canon imageRUNNER ADVANCE 715iF III Series Service Manual page 253

Cause
Fixing temperature is so high that some toner remains on the fixing film after fixing takes place. This residue is fixed on paper as
the fixing film makes another rotation.
Condition
Fixing temperature is too high relative to the temperature required to fix toner onto paper.
Field remedy
1. Select the items in Settings/Registration menu.
[Settings/Registration] > [Preferences] > [Paper Settings] > [Paper Settings]
2. Select a paper source and press the [Set] button
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6. Troubleshooting