Change motherboard and still not defrosting

You have a defrost heater and a defrost thermostat that is wired in series and when the defrost heater is replaced you should replace the defrost thermostat with it. You either can see the defrost heater burned in two or when you check it with an ohm meter it will read infinity (or open). Why did you replace the circuit board before checking the defrost heater? It's the heater that goes out most often; the board usually lasts the life of the refrigerator. I hope you saved it as it was most likely good. Never replace a part without checking with a meter first. Make sure to remove rear cover and clean the condenser coils before putting it back in service to keep from ruining the compressor. This should be done every six months and can affect every system in the refrigerator if dirty. Here is a manual that should be close: https://dl.dropbox.com/u/8010163/GE_Profile_Artica.pdf