Dell P1500 Personal Mono Laser Printer - Manual (Page 60)

The printer top cover is open.
What you can do:
Close the cover to display the light sequence that appeared before the cover
was opened.
What this light sequence means:
The toner cartridge is not installed, or the printer detects a toner cartridge
error.
What you can do:
If the toner cartridge is not installed, install it.
If the toner cartridge is installed, remove it and install a new toner
cartridge (see
Replacing a toner cartridge ).
What this light sequence means:
The toner cartridge you are using was not designed for your printer.