I encountered a laserjet 6l that is printing the paper completely black .I exchanged tthe catriadge with a working one and the problem still exists.Earlier it showed that it had an error like the printer door was open although everything looked fine and I had to open the screws to open the outer case and I realised that the "component" that senses
the printer door is open was out of position and after repositioning it and returning the case ,the error was gone but now it prints the whole paper black.Im lost ,I dont know whatsup.

For a laser printer to work there needs to be a charge put on a photo sensitive drum to repel toner. The laser itself actually changes that charge where the image should be so that these spaces will attract toner. Apparently the charge to repel the toner isn't there.

This process occurs in the toner cartridge itself and so replacing the cartridge should solve your problem. If that doesn't work it could be a part in the printer that has to do with applying a charge to the cartridge itself (which I know next to nothing about) in which case you should have the printer looked at by a service person.

I agree. It's best to have the device examined by a qualified service technician