Some say it is a SE is a sensor error. Others, a Key short error.

The SE error code indicates that you have a stuck key on the control keypad or a failed electronic control board. A technician will need to diagnose and repair this type of failure. Microwaves are inherently dangerous to service. Internal high voltage components can retain a lethal electrical charge even when the microwave is completely unplugged. A microwave shielding test must be performed on the unit to detect any possible radiation leakage following any internal component repair. These are the reasons that I recommend having a technician service the microwave.