Error 55 What is error 55 on my HP LaserJet IIIp printer

Error 55 is a major Engine or DC controller issue.
HP LaserJet Enterprise Printers 55 00 0x and 55 0x 06 Errors

Try spares from salvaged printers. Look for "working pull from damaged unit". Of course reconfirm model and spare compatibility.

Printer Works RM1 9371 000 DC Controller As M775 mfp HP LaserJet DC Controller For M775 Printer RM1 8898 - USD70+shipping

Your 3P printer with the 52 error is going to need a scanner motor assembly.
Have somebody local look at it before you put money into this printer.
Considering its' age it just maybe looking to retire.

A 50.4 code is bad line voltage. Don't plug it in a surge protector.
Any other 50 error is a fuser problem. Turn the printer off for at least 10 minutes. Power it back on. If the error returns, replace the fuser.

Search here -->link.

I would replace the printer if it was giving me that much trouble.

http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?lang=en&cc=us&taskId=125&prodSeriesId=71454&prodTypeId=12771&objectID=bpl11619
This is a link to HP that covers proceedure on cartridge replacement,

Was this happening before you swapped toner?
Is there toner streaks on the drum? Yes? Defective.
What if you stop the printer (open cover) as it passes through the fuser to determine if image is from drum or fuser?
But really....isn't it time to move on anyway???

• HP II/III series most likely cause is the ac power module is defective. Order the HP II/III Service 50 Error Repair Kit. If the quartz fuser lamp is burned out, order the replacement fuser on specials page. Slight possibility of loose connector on DC controller. See tweaking page DC controller could also be defective, but this is very rare.
• HP IIP/IIP+/IIIP most likely cause is burned out quartz lamp. Order the HP IIP/IIIP 50 Service Error Repair Kit. Could also be a failure of the power supply.
• HP 4/4+/5 most likely cause is burned out quartz lamp. Order the HP 4 Service 50 Error Repair Kit. Could also be a failure of the power supply.
• HP 4L/4P/5L/5P/6L/6P most likely cause is a bad ceramic heating element.
• All other printers most likely cause is burned out quartz lamp. In rare instances the ac power supply has failed. Even rarer, the DC controller is defective

I just looked at windows xp and the drivers for HP Laserjet IIIP are included with windows xp.
Go to START, SETTINGS, PRINTERS and FAXES (if using the classic menu, I'm not using the XP menu system or go to the control panel, select classic icons and select Printers and Faxes either way will work. ) once inside the printer and faxes window, choose "Add Printer" Click NEXT, click Local Printer, click NEXT, click the port you will be using (With that old of a printer methinks it will be LPT1:) click NEXT, The driver selection screen will come up, Choose HP on the left hand side of that screen and HP LaserJet IIIP is on the bottom part of the list on the right hand side of the screen (They are not listed in number order like you might assume they are). Click HP LaserJet IIIP and click NEXT, It will ask for a printer name and you can keep the default name, Click either yes or no about the question "do you want to use this printer as the default printer?" and click NEXT, Click if you are going to share the printer and click NEXT, answer "Do you want to print a test page?" click NEXT and it should install the drivers for that printer and print a test page once installed. Then click FINISH on the last page and your printer should be installed.

The 55 error doesn't boil down to one specific problem and most information will tell you the following: 55 ERROR (Internal Communication Problem) 1. Replace DC Controller PCA. 2. Replace Formatter PCA. 3. Replace both at same time. So sounds alot like the main board is starting into a nose dive, but seeing as that particular printer is on the older side, you might be able to find the part on the cheaper side, check out e-bay, they have some HPIII part around. Good luck.