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This frost builds up when the evaporator is overfrosting. Now you can either turn the unit off for 24 hours and make sure all the frost even behind the back panel inside the refrigerator thaws out and see if it comes back, or the other option is to call a technician out to check the freon pressure because this model is known for having a freon leak which will cause the evaporator behind the back panel inside the refrigerator to overfrost on part of the coil but have no frost on other parts of the coil. That would require a technician to recharge the system and repair the leak if that is the case.
Otherwise, if the issue isn't caused by low freon, it's due to the door being left cracked open too long causing too much moisture to enter the unit. Eventually it reaches a tipping point where it can't work properly again without manually defrosting the unit by shutting it down.
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