Lexmark T420 User Reference page 105

Upper front door is open
What this light sequence means:
The printer upper front door is open.
What you can do:
Close the upper front door.
The printer clears the message.
Print cartridge not installed, unsupported print
cartridge, or defective print cartridge
What this light sequence means:
The print cartridge is not installed, not supported by this printer, or
is defective.
What you can do:
• Insert the print cartridge.
• Replace the print cartridge with a nondefective cartridge
supported by this printer.
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