I determined my board was bad when it did not defrost. I checked the heater and t-stat both good. After replacing (and programing) the new board it worked for 4 or 5 days and the refer started to warm up, evap frosted over. I checked the element, good. I checked the termination t-stat, closed. so I forsed a defrost cycle and it defrosted. So I replaced the thermistor in the top right corner thinking it was not sencing the correct temp. 4 days later frost, I again forced a defrost and is now cooling. What am I missing?