There is a vent at the rear of the freezer that allow cold air to cool the fridge however this problem is most lightly caused apparently by failing temperature sensors, seems a common broblem with this model. I have same model and have same problem, but as it's an old machine so am checking all other possibilities before spending money on sensors. I would suggest defrost first but you've done that, move onto remove panel at bottom front and clean out all dust and grime, do same at back the fan on mine was really sluggish with muck, clean that, I turned the machine on and can now see that's running fine, moved onto panel behind top self of freezer, there's another fan there, mine only wants to work intermittently, so unsure about that, I know this helps to distribute air, you can also see one of the sensors below the fan, one other sensor is in the top of fridge in box behind the light, cant see much more that I can do, Im trying to find schematics plans etc. as I believe there maybe another fan and sensor but so far cant find them. Hope you get yours sorted, let us know.