Entered into service mode through temperature control panel and found defrost function not working. Some of the other modes were also inoperable when forced on/off through panel, such as compressor on/off function, but after tapping on control panel housing they would begin to work. All except for the defrost heater element of course, it never worked no matter what I did. I removed freezer panels to gain access to evaporator, disconnected heater element and thermostat, and found heater element within specs (28 ohms), and thermostatic switch functioning properly (open circuit above 42F, closed circuit below 12F). I replaced electronic control board (jazz board), programmed with enclosed instructions, and everything is now operating normally. I'm able to force defrost mode through control panel and watch the element heat up and melt the frost. (Also note, while I had the freezer panels removed I went ahead and replaced the heater element and thermostat. Mainly because even though they were within specs I've read that they have a high rate of failure. Also, who's to say there wasn't also a problem with the heater element when under a load causing the circuit board to eventually fail over time. I'm probably guilty of overthinking that one, but it was so easy and cheap to replace while everything was disassembled. And I believe it will make for a longer lasting repair).