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There are two sensors, on each side of the output tray.
Check that one of the sensors is not stuck or jammed in the "full" position.
The sensor is a flag that flips each time a page comes out of the printer itself. Check to see if that flag is broken, or is malfunctioning, that is what is causing that message to come up. . If it does not move freely with your finger, it may be stuck so carefully try to loosen it. Also, use a can of compressed air to blow the area out, as it often just gets jammed from debris or paper dust over time.