Hi:
My side by side refrigerator started about 8 months ago not to defrost properly. Removing the inner panel and using a heat gun, and checking the defrost thermostat and the defroster bulb fixed the problem for about a week. Since then I replaced the controller board twice and the defrost thermostat. I'm pretty good in electronics, so I defrosted It numerous times by hotwiring the defrost bulb for about 20 min or until the water quit dripping in the pan so I would not have to take the fridge apart. I also connected a hour meter in the back, so I could check, if the defrost was activated. Last time it was manually defrosted was on 8-6-14, today is 8-16-14, and it has not defrosted yet, and the temperature in the freezer is minus 15 deg, and in the fridge plus 60 deg. I have found another, what looks like a sensor on the bottom left side, were the pullout drawer is located, but I could not find it in your parts drawing.
I forgot to mention, that the defrosting cycle, when working, is very infrequent. Any help would be very appreciated!

Hi: richappy
Thank You for your reply, I defrosted it the way you described, and also replaced the "defrost thermostat", the clip on unit on the evaporator tubing. Is that what you called "evaporator thermistor"?.
Like I stated in my letter, I found another sensor in the freezer section, on the bottom left corner, were the pull out drawer is located. I could not find it in the parts drawing, so I wonder, if this is the problem.