The Short Answer
A frost-free freezer runs a defrost cycle every few hours: a heater warms the evaporator, melts the light frost that forms in normal use, and the melt water drains away. When frost starts sheeting on the back panel or the walls, that cycle has failed, most often a burned-out defrost heater or a defrost terminator that will not close. A worn door seal that admits humid air makes it worse. The frost then chokes airflow, so the fresh food side warms even though the freezer still feels cold.
Most Common Causes
- Failed defrost heater. The evaporator never gets warmed, so frost accumulates.
- Open defrost terminator. The safety thermostat that enables the heater has failed open.
- Defrost control fault. The board or timer never triggers a defrost cycle.
- Worn door seal. Humid room air leaks in and freezes on the coil and walls.
- Blocked drain. Melt water refreezes and builds into a block at the base.
Notes by Brand
On Sub-Zero and True Residential built-ins, the defrost heater and terminator are the usual failures, and clearing ice without fixing them only delays the return. On Liebherr refrigeration, a temperature alarm or code often accompanies the frost and points at the defrost or fan circuit. On Thermador and Monogram units, we ohm-test the defrost components to the model spec. On Miele freezers, a worn seal is a common accelerant, so we check the gasket alongside the defrost system.
Symptom Reference
| What You See | Likely Cause | Fix |
|---|---|---|
| Ice sheeting on back panel | Failed defrost heater | Ohm-test and replace the heater |
| Frost returns after clearing | Open terminator or control | Test terminator and defrost control, replace |
| Frost on the door frame | Worn door seal | Replace gasket, reset door alignment |
| Ice block at the base | Blocked defrost drain | Thaw and clear drain, fit heater if specified |
| Warm fridge with cold freezer | Frost choking evaporator airflow | Clear ice, repair the defrost fault |
Frost Coming Back?
Book a diagnosis. Diagnostic and travel fees, if applicable, are confirmed before the appointment and before repair approval.