Initially the freezer became too warm. I saw large amount of ice on the evaporator. Replaced the defrost thermostat and fixed the problem. One year later, ice started to form on the evaporator again. The fridge keeps running even when the set temperature reached inside the freezer. The defrost thermostat appears to be fine. I can see defrosting still works, but it just doesn't last long enough to clear all the frost. Eventually the frost near the fan inside the freezer turned into ice and it starts to accumulate...

I think the issue may be caused by either thermistor or control board. I checked the resistance of the thermistor over temperature and it seems correct. Is such a check good enough? How can I check the control board without buying another one to try out?

Thanks for all the inputs!