I have the same problem as above. I found the sensor board over to the left in the same compartment as the paper tray, the only problem was that it was smashed due to the paper tray being jammed in many times.
I tried to source one with no luck.
I took the board out and all it had is three push buttons, and an opto coupler. The coupler was smashed, the board broken. I repaired the board correctly, obtained a opto LED and Opto receiver, soldered them in and all worked fine.
If there is still a problem with yours, check the plug socket connector for the sensor board.