Why is my LG 3 door fridge that is 3 years old not cooling?

I have a LG LFX28968st/06 three door refrigerator that has stopped cooling. The compressor did not seem to be kicking on so I changed the start relay. This brought the compressor on but the freezer was still warm. The error code on the display plan came up as FF (freezer fan) which I replaced. I also cleaned out the coils as best as I could in the back. Now both the freezer and fridge will not get below 50 degrees. The freezer fan, compressor and fan that blows on the coils in the back all are working. Does anyone have any ideas? Repairmen in my area charge $150 just to diagnose then extra for work to be done.