HP Laserjet 4 repair guide. Error codes, service manual.

The Hewlett Packard Laser jet 4 series printers are the best printers ever made in my book. I bought my HP Laser jet 4m plus back in 1994. I never had a problem with it until today. After 16 years of service the High voltage power supply finally gave out. The quality of this printer is way too good not to fix it. The bottom line is they don’t make them like this anymore! I wrote this article to help others fix their HP printer. It is real easy and really cheap!

My problem started with my HP Laser jet 4m plus printer after replacing a toner cartridge. The display showed an error 16 – Toner low message. The prints started to look bad and faint. The toner coverage was getting weeker. I thought maybe the fuser was going bad. I researched this error code all over the web and found some good info on this problem. Hopefully, it did not take you long to you find this article.

The error 16 – Toner low message would not go away after cleaning the sensor contacts as mentioned in some of the repair articles. (This is a good reference link for you). My problem turned out to be a bad high voltage power supply. I researched prices on these and you can get one for around 40 bucks if you are lucky. I bought another HP Laserjet 4m plus with a low print count on ebay for a parts machine at about 50 bucks. I have taken good care of my printer and it is in great condition. My printer had only 55,000 page count which is nothing for these work horses.

After hours of research online, I could not find any step by step instructions on how to replace the high voltage power supply on a HP Laserjet, 4m plus printer. I did not even know where it was located. The high voltage power supply is real easy to swap out. Follow these instructions and your back in business.

Step 1
Remove the toner cartridge.
Remove the paper tray.
Turn the printer upside down as shown.

HP LaserJet_4m plus upside down, with paper tray removed.

Step 2
That black box you see is the High voltage power supply.
There are only two screws you need to remove.
The right side screw and the center screw.

Removing the high voltage power supply (HP Laserjet 4m plus)

Step 3
Press in on the two retaining tabs on the enclosure box and gently lift out.
Tilt the front section back and up to release from the tabs as you lift.
Notice the power connection circled in the next image.

Tab 1
Tab 2
Gently squeeze the enclosure at the tabs, tilt back and lift up and out.

That was easy!
Here are more detailed images. Probably more than you need but what the heck?

HP high voltage power supply removed

Step 4

Replace the power supply module with a new one. You can get one here at this link. www.printerworks.com Press in on the two retaining tabs on the enclosure box and gently back in. Insert the back section first. line up your holes and power supply plug
Notice the power connection circled in the above image. This connection aligns perfectly as you replace the unit with the new power supply.

The factory part number for the High voltage power supply is RG5-0969
Factory part number
Power connection
Align the holes and tabs. Gently squeeze the enclosure at the tabs, tilt back and press in.
Tighten up the screws and your done!

When you power back up the Error message – Toner low should be gone. If not you have other problems! Sorry I cannot help you. This fixed my HP laser jet 4m plus printer. Thank you Hewlett Packard for making a great product and easy to service!

I hope you find this blog helpful.
Here is a link I found where you can buy the high voltage power supply for $29 bucks. I am sure there are more HP parts suppliers but this is a good price! www.printerworks.com

Please send me a note if you have any questions!

Glenn Clegg

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