I have also had a problem along these lines, 3 weeks after the warranty ran out. An error appears that says reload paper, but there is no paper in there! I have read about cleaning the sensors, also downloaded the service manual (but it won't open the files on my mac) I went to take the back of the printer off to expose the sensor but the bolts are using a different screw driver. What have you all done to get these screws out?
They are called Torx or T screw drivers . HP use essentially 6, 8, 10, and sometimes T 12 screws.
Many cases , you can use hex wrenches place of screw drivers. Torx