What does the control board do? How can I test if it's defective?

My problem is a cold freezer hot refrigerator. I think it's a defrost problem. The evaporator fan is running. The condenser is running. Frost build-up in the freezer. I removed everything from the box. I turned the power off and using a heat gun I defrosted the coils. The evaporator pan filled several times with water. I cleaned all of that. I restarted the refrigerator and within 2 hours it was at the desired temps. It's been running perfectly for 36 hours with no problem. I can hear the evaporator heater turn on and I've gotten some water in the evaporator pan. I'm questioning the repair man's diagnosis. The first thing he did was hook up (looked like an extension cord) to a wall outlet then he plugged in the refrigerator cord to it and had an electronic loop tester and then told me the control board was bad. That was it -- no further testing. So, like I said above what does the control board control in this refrigerator and what is the proper test for discovering if it's defective.