I have to shut down my refrigerator every 8 to 10 days to defrost it. I open it and found only one defrost heater, which I tested and verified it is fine. I dont know whether a defrost heater is missing or if the defrost timer is not working properly. I have two questions: 1. How many defrost heaters should have been installed at the factory in my refrigerator, one or two? 2. Should the fan being working during the defrost cycle?