Jam B (Cover B) - Epson AcuLaser C4000 Owner's Manual

When a paper jam occurs repeatedly at the paper cassettes (Jam
AC or Jam ABC), jammed or crumpled paper may still be inside
the paper cassettes. Remove the paper cassettes, then remove the
black unit that is located on the top part of the inside of the
cassette slot, and check whether any jammed paper is there.
When the paper jam is cleared completely and cover A is closed,
printing resumes from the page that jammed.

Jam B (cover B)

When there is paper jammed at the duplex unit, the LCD panel
displays the Jam B error message.
Follow these steps to clear the jammed paper.
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Troubleshooting