HP LJ4 Paper Jam on Start-up

I need to service a HP laserjet 4 that gives a paperjam error on start-up, which I assume is due to a bad sensor, or a bad rear door switch. I have not been able to find any repair information on how to verify whether the problem is with the sensoror the rear door closure switch. I don’t know how to identify the sensor, or remove and replace it.