Kyocera FS-2020D "Add Paper to MPTray" Error Message

I have a Kyocera FS-2020D with an error message that states "Add Paper to MPTray". There is paper in the Multi Purpose tray, but it is not detecting it. I've tried re-inserting the paper stack, power cycling the printer (with the power switch and unplugging and plugging it back in), I've checked settings (set to MP tray) on both the printer and the computer it's printing from.

I think it may be the MP tray paper feed sensor is dirty, but I can't find it to try and clean it. I've checked the service manual, but it doesn't give a location of the sensor for the tray, it just shows the sensor connected to the power source unit.