Modern refrigeration is designed to defrost itself automatically, melting the thin layer of frost that forms on the evaporator coil during normal cooling. When frost keeps building up, that automatic cycle has stopped working. Left alone, the coil ices over until airflow is choked, and then the fresh-food side warms up even though the freezer looks colder than ever.
The Most Common Causes
- Failed defrost heater: the heater that melts coil frost has gone open, so ice accumulates cycle after cycle.
- Defrost sensor or thermostat: the sensor that triggers defrost is reading wrong or has failed, so the cycle never starts.
- Defrost control or board: the logic that schedules defrost has failed.
- Door seal: a worn gasket lets in humid air that feeds heavy frost.
- Drain iced shut: a blocked defrost drain lets melt water refreeze into a block.
Notes by Brand
Sub-Zero: built-ins use microprocessor defrost logic, so a stuck cycle often traces to the defrost sensor or heater, and the control logs help confirm it.
Viking and Thermador: a defrost heater that has gone open is a frequent cause of coil icing on professional built-ins.
Liebherr: frost with a temperature alarm usually means the defrost sensor or a fan is at fault.
Monogram, Jenn-Air, and KitchenAid: defrost heater and thermostat failures are the common culprits on these built-ins.
Symptom-To-Cause Reference
| What You Notice | Likely Cause | Our Repair |
|---|---|---|
| Coil iced over, fridge warm | Defrost heater failed | Test and replace heater |
| Frost returns after clearing | Defrost sensor or control | Test circuit, replace part |
| Heavy frost near door | Worn door gasket | Replace gasket |
| Ice block at drain | Blocked defrost drain | Clear and heat-check drain |
| Freezer fine, fridge warm | Coil frost blocking airflow | Clear coil, repair defrost |
Frost Keeps Coming Back?
Book a diagnosis and we will fix the defrost fault, not just the ice. Diagnostic and travel fees, if applicable, are confirmed before the appointment and before repair approval.
Frequently Asked Questions
Why Is Frost Building Up in My Freezer?
Can I Just Chip the Ice Out Myself?
Does Frost Buildup Affect Cooling?
Related
Refrigerator Repair
Defrost system service.
Not Cooling
When frost leads to lost cooling.
Water Leaking
When a blocked drain overflows.