Epson EPL-N1600 Reference Manual page 149

Illegal Aux I/F Card
This message means that the printer cannot communicate with
the installed optional interface card. Turn off the printer and
remove the card, then reinstall it.
Illegal DIMM x
The printer cannot read the installed optional DIMM. Turn off the
printer and remove the DIMM.
Image Optimum
There is insufficient memory to print the page using the specified
print quality. The printer automatically reduces the print quality
so it can continue printing. If the printout is not acceptable, try
simplifying the page by limiting the number of graphics or
reducing the number of fonts or font sizes.
Turn off the printer then resend the print data.
Turn the Image Optimum setting off on the Printer Setup menu
in the printer driver or SelecType.
You may also need to increase the amount of printer memory. See
Chapter 3, "Options" for more information on adding memory.
Insert Duplex Tray
The duplex tray is not properly inserted. Re-insert the duplex tray.
Troubleshooting and Maintenance
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