Assessing the cause of freezer coils icing up
Like the OP said. &quot;It's a sealed system.&quot; If it was a leak it wouldn't &quot;remove heat&quot; at all. (fridges don't cool, they remove heat.) Automotive AC's DO leak, slowly, because they use rubber hoses/lines. So forget about the &quot;low refrigerant&quot;, that ain't it.
The problem is one of three things. First (and most likely) the defrost timer. It's usually located in the plastic housing in roof of fridge compartment. Look for a larger hole that does not have a Phillips screw in it. Use a flathead screwdriver to turn the flat head looking plastic piece you'll see inside that hole. It's like a timer you use for your Christmas lights for example. Turn it until you hear a loud &quot;click&quot;. Listen for your compressor/motor to turn off. If after about 5-10 minutes your coil starts dripping water/black heater coil bar gets hot (the header runs along coil; don't touch it, it gets real hot) then your timer's shot. Go online with your fridge model number and order that cheap part. Remove the screws that hold that plastic cover on, unplug the old put in the new, WITH THE FRIDGE UNPLUGGED. Turn the new timers screw until 2 clicks when u install it so it's in cooling mode.
If it does not heat up it's probably the defrost thermostat (a little disc mounted by coil.) Test with multimeter. It should have continuity when it is at freezing temperatures and should change to open circuit when the temperature rises above approximately 37 degrees F.
If that checks out then it's the heating coil rod. Double check the timer/thermostat before concluding it's that because it's a bit of a job to replace and least likely problem.
There is also a slight chance that it could be cause by a bad door gasket but you won't see major solid freeze up, just excess, making the unit work harder and run longer than it should. Here's a decent video on this issue. https://www.youtube.com/watch?v=2kNfNfHTgCo
He does keep calling the timer &quot;the thermostat&quot; but corrects himself in captions.