Our fridge will work fine for about 5 weeks and then the return duct ices up. The freezer works well. The defrost function seems fine and there is no frost build up on the coils. Both fans are working. The freezer drain is open. Air is now blowing through the damper into the fridge, although I did just defrost the unit to clear the return duct so everything should work for a while,

I have read though the "Freezer is fine, fridge too warm" thread but those posts seem to start and finish with defrost problems.

What would be making the return duct ice up like that? Should I just blame my kids for leaving the door open as this happens intermittently?