The five most common causes
1. Door seal isn't sealing
Most overlooked. A worn or dirty gasket lets warm room air in. The compressor runs constantly trying to compensate, the freezer climbs to 15–20°F, and food slowly thaws. Quick check: close the door on a dollar bill at several points around the frame. If you can pull it out easily, the seal is weak.
2. Defrost heater failed
If you open the freezer and see a thick layer of frost on the back wall — not just a light dusting — the defrost heater or thermostat is bad. The coil is icing up, airflow drops, the freezer warms. We test the heater with a meter; replacing it is a same-visit repair on most units.
3. Condenser is blocked
The condenser is the radiator-like coil that gets rid of heat. On modern units it's at the bottom or back. If it's coated with dust, lint, or pet hair, the unit can't release heat — so the freezer slowly warms up even though the compressor is running fine.
For built-ins this is the #1 cause we see. The grille at the top of the cabinet collects everything. A cleaning often solves it.
4. Condenser fan failed
Same effect as a blocked condenser. The fan that pulls air across the coil isn't moving. Either the motor failed or something is jamming the blade. We can usually hear this within seconds of the unit being open.
5. Sealed-system problem
Low refrigerant or a compressor that's losing capacity. Less common than the four above. Diagnosis takes longer — we measure compressor amperage, check for frost patterns on the evaporator, and confirm with temperature readings. If the sealed system is the problem, we'll explain repair vs. replace honestly.
If the fridge is also warm
Then we're definitely looking at the cooling system, not the defrost path. See refrigerator not cooling for that scenario.
Built-in column freezers
Sub-Zero and Thermador column freezers (and Miele MasterCool) have their own diagnostic patterns. For those, see built-in refrigerator repair.
Save your food while you wait: Pack the freezer tight — frozen mass holds cold longer than empty space. Don't open it. If you can, transfer the most expensive items to a neighbor's freezer or a cooler with bagged ice. We'll get there as soon as we can.