It is an error with the laser scanner unit
1 Press ( SELECT button) to continue.
2 Turn the printer off and then on.
3 Reseat the connectors between the
laser/scanner and dc controller
PCA (J83 and J84).
4 Replace the laser/scanner assembly.
See Laser/scanner assembly.
5 Replace the dc controller PCA. See
Dc controller PCA.
If nothing from above works and the printer is no longer in warranty, it is possible if the printer is being used in a very dirty or dusty enviornment that the lens and optics of the laser/scanner are so full of debris that the beam detect error occurs. You can remove the top cover and the big black box in the center is the laser scanner assembly. Remove the screws securing the top cover of the laser scanner and remove and clean up all the optics to include the main mirror in the front. I usually use a damp q tip and for the mirror I slide up from toner cartrdige shutter area and observe from the top as it cleans the mirror. Removing the complete assembly on this model is not easy since there are a couple of cables going to the dc controller which are difficult to remove so removing the top of the assembly is easier.
Here is the service manual:http://www.feedroller.com/Store/Service%20Manual/LaserJet%204250-4350%20(%204200-4300)%20sm.pdf
Click on this link or copy and paste the complete link into your browser.
If I could be of further assistance, let me know. If this helps or solves the issue, please rate it.
I’m happy to assist further over the phone at https://www.6ya.com/expert/joe_8b8c2cd6ce148309