Dell P1500 Personal Mono Laser Printer - Manual (Page 61)

The toner cartridge you are using was not designed for your printer.
What you can do:
Install a toner cartridge made for your printer (see Ordering replacement
toner cartridges ).
What this light sequence means:
The printer is in the Ready mode and Hex Trace is active.
What you can do:
Advanced users can use Hex Trace to help troubleshoot printing problems.
After resolving the problem, turn off the printer to exit Hex Trace.
What this light sequence means:
The printer is in the Ready mode, Hex Trace is active, and the toner in the
toner cartridge is getting low.