asked May 22, 2013 at 3:43pm
Hp HP LaserJet 4

HP Laserjet III - 12 Printer Open...

Help, Help, Please...

I have an HP Laserjet III. It really is an original III, with no modifiers When I turn it on there is no sound. It goes through “Self Check” and finally displays “12 Printer Open”.

Problem began suddenly in the middle of several hundred pages, right as I needed to print my tax return, stopped with “12 printer open”.

Immediately following power on, Ready, Manual & Online leds light up, display “05 Self Test”…..Lights go out…..12 Printer open”, never see “warming Up".

I have observed the fans, both upper and lower, neither are moving.

I am pretty sure the motor is not doing anything. I sort of remember originally hearing a motor sound, like a sync or “rollover alignment”.

The fuse in the DC power tests good, the pins (right side under plastic and metal covers) for +24v, +5v & -5v are all 0.0v.

I do not believe the DC power is getting any input power as measured on the 2 outside pins (of 3) very near the fuse.

My guess is the AC power module is defective since at least 1 fan is plugged directly into the module (and does not spin).

I have tried a second toner cartridge, no change.

I have extended the “Cover interlock bar” (using multi-layers of masking tape) that is attached to the cover itself, no change.

Only standard consumables have been replaced including toner cartridge & fuser cleaner/wipe and the ozone filter, since new.

As I read the combined service manual, there is an AC power module, this is where the line-cord plugs in and at least one fan (the upper one) plugs in for its power.

Then there is also a DC power supply which has a fuse in the lower front edge and also the micro switch for the cover interlock. Then there is a High Voltage power with the print density. And again there is also a DC power controller (somewhere under the chassis bottom.

So with a total of 4 power something’s that must be powered initialy by AC, I am thinking it is the first in line: AC Power module” problem.

So my bottom line question is, is this logical and what do you recommend. Alternative ideas welcome.


If the printer "thinks" the cover is open, then the POST is halted and you will not see any other movement. If the micro switch for the interlock is bad it must be replaced.
Good luck with that.
by unknown on May 22, 2013 at 4:21pm Add comment
I always have replaced the DC power supply and that's fixed it every time. If you need one
by moe on May 22, 2013 at 4:49pm Add comment
I thought so, just wanted a bit of confirmation...Order is on its way...


by SkyBlueflyer on May 22, 2013 at 5:33pm Add comment
just restoring subscription
by unknown on May 25, 2013 at 10:43am Add comment

I replaced the DC power supply as you suggested and nothing has changed since the original post.

So I guess you will next recommend changing the AC power supply as I first suggested?...or?

by SkyBlueflyer on May 27, 2013 at 1:38pm Add comment
Let me explain the workings of everything. The AC power supply has 2 outputs. One is where the fuser plugs in, the other goes to the DC power supply. There is a small board on top of the AC power supply that the fan plugs into. I wouldn't read much into the fact that it isn't turning because there were 2 versions of that board. One ran the fan at idle and switched to high when printing. The other only ran the fan at printing. Since it still has a 12 door open, I would suspect the DC controller. The DC power supply plugs directly into the DC controller and powers it and the controller distributes the DC throughout the printer and monitors all the sensors, switches etc. and generates the appropriate fault code to the Control Display Panel.
by moe on May 28, 2013 at 11:52am Add comment
This is my first repair so I don't know much about laser printers, I am trying to understand and most important, to get it working.

Ok, so if it get this correctly, the next step is the DC controller, which is in the bottom of the printer...?...and pretty much runs the show?

What other parts, in order of likelyhood could it be?

Thanks for your help,


- SkyBlueflyer
Well, with the replacement of the DC Controller, I can print a ""Test Page"", all LOOKS good, I have not yet placed on line to see actual printing from an application.

And so...Moe... While I did not place much on the upper fan turning origianlly, what I now see (with everything else seemingly (sp?) working is the upper fan "Quivering", it just does that until I "touch/push" in the proper direction. Until it rotates peoperly there is a big OZONE smell.

So Do I need to replace the fan or?......
by unknown on Jun 2, 2013 at 4:15pm Add comment