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So F31 could be caused by the temp thermistor being faulty, or it could be a bad wire connection between the sensor and the main control board, or you could just have a bad main control board. The main control board is usually the cause for this error but you can check the resistance through the thermistor with a multimeter if you have one to make sure it reads 1080 at room temperature and it didn't come faulty. If that checks good, then follow the wires from the sensor to the board and make sure nothing is loose or broken. And if all that checks good, the main control board would need to be replaced I'm sorry to say.
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