Error 51 - HP Laserjet 4M+

Hello,

My HP Laserjet 4M+ is throwing an Error 51 when attempting to print. I have replaced the laser scanning assembly (Original part RG5-0911) with RG5-0449. I'm reading conflicting information about those part numbers being interchangeable. Can anyone confirm?

Nonetheless, that did not fix the issue. I replaced the DC controller board RG5-0996. Problem still exists.

Either the scanning assembly is incorrect or there is another problem. I didn't want to purchase another scanning assembly if these are interchangeable. If they are, what's the next troubleshooting step?

Thanks!

EDIT: Update - Those parts are not interchangeable. I put the original scanning assembly in and the printer works. :) Looks like it was a bad DC controller board.