Un-tried information for HP 4500/4550 Printers:

Accessing the service mode:

1: Turn the printer off
2: Hold down the (CANCEL JOB) and (SELECT) keys and turn the printer on, 
   then wait until the display goes blank and the three LED's come on
3: Press the right side of the (MENU +) key, then press the (SELECT) key
   SERVICE MODE will appear on the front panel briefly 
   followed by two rows of asterisks. 
4: The printer will go through its normal start-up cycle, 
   and then display SERVICE MODE on the front panel again. 
   Now you are in SERVICE MODE
5: Press the ITEM key until the DRUM KIT COUNT or FUSER MAINTENANCE COUNT appears
6: Press VALUE +/- to change to zero
7: Press SELECT to save
8: Press GO; the printer should return to READY

9: YOU MAY NEED TO REBOOT THE PRINTER, or perhaps not