Why does my sub zer refrigerator have frost on the inside?

Hi,didn't realize it was a sub zero,if the frost or ice is on the inside back wall of the fridge section on the left side you could have a freon leak in the evaporator coil,if that's where it is and if it's under 12 years old the coil is covered under warranty,if you write back tell me where the frost is and send the model number and the year it was made and i can help you out better,if it's a side by side open the freezer door and look at the top hinge and you'll see a tag there with all your info,if the freezer is on the botom open the fridge door and look at the top hinge and the tag will be there.also like i said on one of your other questions frost can also come from the door being left open,it's not from the controls being turned up to high,controls should be on 5 or 6 or if it's digital freezer on 0 and 37 in the fridge section.good luck and hope this helps you out.one more thing with sub zero,very important to clean out the coil and cabinet,you can shut down the machine or unplug it,there's a quick disconnect up top behind the white cover on the left side.get a shop vac with a brush attachment on it and clean the face of the coil off and all the walls and ceiling,next get a towel and cover the opening where the front grill goes,of course this is with the front grill off and the left side cover off,cover the opening and set the shop vac up to blow,stick the hose under the towel and blow through the coil,lift up the towel and vac up all the dust that blows out,do this a few times and then go on the other side and blow through the coil where the fan blade is,also grease sometimes gets up on the coil from cooking,the grease gets airborn and with the fan blowing up in the top the grease can get on the coil,if so get some grease remover and spray the coil down and if it's on the front grill spray that down too,you have to clean the coil at least once a year or your gonna have problems with it,the towel will keep the dust from blowing into your kitchen,it works for me.hope this helps and good luck with your subby,i think it's the best machine out there.