"I have tried to reseat the EP-L cartridge and opened and closed the fuser door many times, but can't seem to get this printer to work."
(From: Erik Welander (firstname.lastname@example.org).)
I have written a detailed description of how to repair the "Error 12 OPEN or no EP-L" problem of the Lasejet IIP that seem to be very common on this printer (cost of approximately $1 for the capacitor). This, and some general maintenance advice can be found at: HP Laserjet Maintenance and Repair.
(From: Terry (email@example.com).)
Sometimes the spring tabs on the left side of the printer that are the sensor for the cartridge get bent or are dirty. Also you get this error if the fan is not working.
(From: Dave Lee (firstname.lastname@example.org).)
Most times I find that its the Density PCA that causes the problem. Sometimes its just dirt/dust in the two sensors on the right side above the toner cartridge, blow these out and give it a try. Density Board is avail through most laser parts houses, these are the ones I use: