HP Laserjet 4 and 4 plus series service mode

1. Hold down the [On Line], [Form Feed], and [Enter] keys while powering ON the printer, until the ON LINE and FORM FEED indicators are both illuminated and the Display Window is blank. (If the Display Window reads 05 SELF TEST at this point, the keys were released too soon. Repeat this step until successful.)
2. Press the [Form Feed] key, then the [Enter] key. The message SERVICE MODE is displayed briefly, then the printer automatically begins a 05 SELF TEST. After several seconds, the ON LINE and FORM FEED lights turn OFF.
(The printer may display 02 WARMING UP if it has not warmed up completely.) After the printer has warmed up and passed the self test, SERVICE MODE is displayed.
3. Press [Menus] once to display SERVICE MENU.
4. Once it says SERVICE MENU press ITEM to scroll through service mode items.
5. To exit the Service Mode press the [On Line] key.