This microwave uses a Humidity Sensor that detects moisture during cooking to determine when they are done. The microwave checks the humidity and if it detect an open or short in the sensor; it will generate this error. In my research the sensor is a common failed part. Do check the wiring also for faults.
To access the sensor; the outer metal cover of the microwave will need to be removed. It plugs into a socket so no soldering is needed.