Thanks for that information.
On this one, please check to see if the air vent(servo controlled) is opening and closing or if it seems almost closed all the time no matter where you set the fridge temp.
Usually this is the cause for your problem considering the defrost system and the cooling in the freezer is fine.
More than likley you will need to replace that vent and its door motor, it all comes as a one piece unit that plugs in, pretty easy to replace as well;-)
The best place to get that air gate as I call it is: http://www.4lg.co.uk/
Once you have the part, and if you have rated my service kindly, you will be able to re connect with me for 30 days so we can get this problem solved quickly.
In any case, I will stick with you until this is complete.
Please rate my service kindly.
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Once you rate, I will be notified and will add your question to my priority list so you can reach me anytime until this problem is solved.
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Thank you and have a great day!