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Your problem is likely to be the "Density Control PCB" that if you had the top cover off the unit, it would be a little board with a lot of wires (mainly purple, I think) going to it. It is in the front on the top.
About $40. Not saying that's it for sure, but I've seen it three times in the last 4 months.
(From: Frank Reid (reid@indiana.edu).)
I agree, but it could also be the assembly of photoelectric sensors and their wiring harness (about $25). Before replacing components, clean the sensors with compressed air and re-seat their connectors on the DC Controller board.
(From: Greg Smith (n6nyx@cox.net).)
Check the 10 uF electrolytic capacitor on the Density Control PCA board to clear Error 12 Open or No EP.
The capacitor on my printer went open-circuit big time! It started off with an intermittent error message. Percussive maintenance brings it back for a while, then it dies for good! The capacitor is only rated at about 16 V. I put in a 10 microfarad, 50 volt unit to replace it and still had plenty of room to clear obstructions after I bent it over a bit!
(From: Tony Duell (ard@p850ug1.demon.co.uk).)
I am not sure how the printer detects that the cover is closed - there are 2 obvious ways : from the +24 V interlock switch or from the toner cartridge microswitches. Since, I believe, it'll claim the cover is closed even without a toner cartridge in place, I think it's the former.
OK, things to check:
Firstly, there should be a little peg on the cover (on the right hand side under the edge) that goes into a slot on a metal housing on the LV power supply module. This presses a microswitch inside that housing. This peg often breaks off.
If that's not it, then take off the bottom cover from the printer and stand the printer on one side. It's safe to power it up like this. Connect a voltmeter between the 2 outside pins (1 and 14) on J212 (a long 14 pin socket on the DC controller board). Power up. If you don't get 24 V here, you have a LV power supply problem.
Now connect the meter between pins 2 and 13 on the same connector. If you don't get 24 V now, then the problem is almost certainly the microswitch in the LV power supply, or the peg that operates it, or something like that. You can probably fix that by taking out the LV power supply, taking it apart and examining the switch and its connections.
If you do get 24 V there, then assuming the printer uses the +24 V sensing as the method of detecting when the cover is one, the problem is possibly R817, R818 or C760 on the DC controller board. If it's not those, then it's IC218, a custom microcontroller.
As I said, it _might_ use the toner cartridge switches to detect when the cover is closed. Take off the upper casing, the frame round the front, and the LV power supply module. On the side of the power supply there's a PCB with the paper sensors, the clutch solenoids, and the cartridge switches on it. Check the latter (SW301 and SW302) and the spring leaf that operates them.