more than likely you have a bad defrost sensor. you can check the defrost board yourself.remove the outer door, ther will be a small at top inside of this one,remove it also. be careful, this is high voltage. unit must be running in haet, locate the defrost board, you will see a pin marked FRC,there is a yellow wire wich should be on NORM.pull this wire and put on FRC pin. should go into defrost, if it does sensor is bad, if not board is bad.if you have to replace board, i would replace sensor also. board is most expensive of the two.