Sub-Zero Freezer Repair

Frost building up in a freezer is almost always the defrost system, usually the heater or the bimetal thermostat (a small heat switch clipped to the cooling coils). It's rarely the sealed refrigerant system. A freezer that can't hold temperature in an Arizona garage in July is often doing nothing wrong. It's rated for room temperatures it never sees out here.

On a Sub-Zero freezer column, the sealed refrigerant system (which has to be opened with special tools and training) serves that cabinet alone. So a freezer that won't hold temperature isn't the airflow-and-damper problem it would be on a combo unit. The usual causes come in a predictable order. First a condenser packed with dust, then a defrost circuit that's quit, and only after that the sealed refrigerant system itself. The defrost problem is worth understanding because it looks like something much worse. When the defrost heater or its termination thermostat (the switch that ends each defrost) fails, frost builds up on the evaporator coil until air can't get through. The freezer warms slowly while the compressor runs without stopping. That looks exactly like a dying compressor, but it isn't one, so finding the real cause matters more than the symptom. Freezer-column door gaskets take more stress from the cold than the ones on the fridge side, and they go hard sooner. Frost along one edge of the door is usually the gasket, not the refrigerant system.