Frost taking over a Sub-Zero freezer almost always means the defrost cycle has stalled. The defrost heater, defrost thermostat, or the control that runs the cycle has failed, so the evaporator never clears and ice accumulates until airflow chokes off.
A working freezer frosts a little and then melts it away during a periodic defrost cycle. When frost builds unchecked, one of the three defrost components has failed: the defrost heater that melts the frost, the defrost thermostat that tells the system when to run, or the control board that schedules it. Diagnosing which requires testing the circuit, but the symptom, a coil buried in ice, is unmistakable.
The consequence of a stalled defrost is a cascade: as frost fills the evaporator, airflow drops, the freezer struggles, and eventually the fresh-food side warms too because the two sections share circulated air on some models. Owners often notice the warming before the frost, which is why a warm Sub-Zero and a frosted freezer are frequently the same underlying problem.
A secondary cause is a door gasket that no longer seals, letting humid room air in to feed the frost. We check the gasket and door alignment along with the defrost circuit. Once the failed defrost component is replaced and the coil manually cleared, the freezer returns to normal, and the fix is a modest one on a Sub-Zero.
Quick Reference
| Cause | How It Shows | Fix |
|---|---|---|
| Defrost heater failed | Coil iced, freezer warms | Replace the heater |
| Defrost thermostat | Cycle never triggers | Replace the thermostat |
| Control board | No defrost scheduled | Diagnose and replace board |
| Door gasket leak | Frost near the door | Replace the gasket |
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