Ge fridge/freezer defrost won't run

I am diagnosing my why my freezer( gss25iynxhfs) won't defrost. I checked the ohms on my defroster and thermometer and they appear to be good. I replaced the thermometer as a precaution and it was cheap. The defrost heater reads 40 ohms which appears correct. I put the system into force defrost mode (test 14) and I don't feel the heater getting warm at all. Should I look into replacing the main board or the heater?