Epson AcuLaser C4000 Owner's Manual page 332

Install Photoconductor
There is no photoconductor unit installed or the photoconductor
unit is not installed properly. Turn off the printer and install the
photoconductor unit if it is not installed.
If a photoconductor unit is already installed, turn off the printer,
open cover A, then open cover D. Remove the photoconductor
unit, then reinstall it properly. Close covers D and A. The error
clears automatically if the photoconductor unit is properly
installed.
Install TransferUnit
There is no transfer unit installed or the transfer unit is not
installed properly. Turn off the printer and install the transfer unit
if it is not installed.
If a transfer unit is already installed, turn off the printer, open
cover A, and remove the transfer unit. Then reinstall the unit
properly and close cover A. The error clears automatically if the
transfer unit is properly installed.
Invalid AUX I/F Card
The printer cannot communicate with the installed optional
interface card. Turn off the printer, remove the card, then reinstall
it.
Invalid HDD
Either the optional Hard Disk Drive is damaged or it cannot be
used with this printer. Turn off the printer and remove the Hard
Disk Drive.
Invalid IPDS
Turn off the printer and replace the IPDS module.
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Troubleshooting