The short answer
A freezer that runs continuously and still drifts warm has usually lost its seal or its ability to reject heat, not its refrigerant. Both of the likely causes are owner-checkable and neither needs a part. Put a thermometer in the cabinet overnight rather than trusting the display. A freezer holds food safely at zero degrees Fahrenheit or below, and knowing how far off it is changes what you check. On a Sub-Zero freezer the cause that ranks first is condenser coil blanketed in dust.
What you observe
- A solid sheet of frost on the rear wall of a frost-free cabinet — The defrost system has stopped clearing the coil.
- Food is soft or partly thawed but the cabinet is clearly running — Cold is being made; it is not keeping up with the losses.
- Heavy frost building far faster than it used to — Humid air is entering, which points at the seal.
- NEAR MISS — the cabinet freezes properly but leaks water — That is a drain fault and it belongs on the water-leak page.
Causes, most likely first on a Sub-Zero freezer
- Condenser coil blanketed in dust — The coil cannot reject the heat the compressor removes, so the machine runs continuously without ever getting the cabinet down. It ranks first on a Sub-Zero freezer because a built-in installation breathes through a narrow cabinet cut-out, so anything airflow-related surfaces here long before it would on a free-standing machine.
- Door or lid not sealing, or left ajar — Warm humid air enters continuously, frost builds fast, and the cabinet never recovers between door openings. Second on this platform: a cabinet opened often, or one whose lid is held up by a packet, never gets the chance to catch up.
- Defrost stalled, evaporator iced over — On a frost-free cabinet the heater stops clearing the coil, frost builds into a sheet and airflow stops while the compressor runs on. Ranked here because it applies to frost-free cabinets, and on one it is a leading cause that builds quietly over weeks.
- Evaporator fan failed on a frost-free cabinet — With no air movement the coil freezes into a block while the rest of the cabinet warms, which looks identical to a defrost failure. Last, and the reason to work down the list rather than guess: it mimics a stalled defrost exactly, so the fan has to be heard before the defrost is blamed.
Check in this order, and why
- Feel the coil area for heat and look for a dust blanket — A covered coil is visible, and it is the cheapest cause to fix.
- Run a sheet of paper round the seal and check the lid or door closes on its own — A seal that does not grip paper is admitting air constantly.
- Empty the cabinet and look for a solid sheet of frost on the rear wall — A sheet rather than a dusting is the answer.
What an owner can safely check
- Unplug the cabinet and vacuum the coil and its grille thoroughly — Twice a year in a house with pets, or in a garage or outbuilding.
Tools: vacuum, soft brushAbout 25 min
- Clean the gasket and the sealing face, re-seat any lifted section and clear anything obstructing the door — Check nothing inside is holding the lid a fraction open.
Tools: cloth, paperAbout 15 min
- Empty the cabinet, switch it off and leave it open for a full defrost — Cold restored afterwards confirms the defrost system.
Tools: towelsAbout 45 min
- STOP HERE — the next check means a meter, a panel, or both — The ranking above tells you what to ask for; it does not make the remaining tests owner work.
Before you open anything
Safety
Unplug the cabinet or switch off its breaker before removing any interior panel — a capacitor can hold a charge after the power is off. Never cut, open or attempt to recharge a sealed refrigerant line, and get help to move a loaded chest cabinet.
When to call a technician
Call when the owner checks are clear, and immediately if you see oil at the compressor or smell burning — oil outside the tubing means the system is open. On a cabinet of this class, insist the cause is named before any part is ordered: the parts are expensive enough that a guess is not worth making.
Nearest pages of this class
What this page does not cover
This is the generalized page for a Sub-Zero freezer. It does not carry anything model-specific: no part numbers, no wiring colours, no panel sequence and no torque figures, because those differ between families sold under the same badge. Where a family- or model-level page exists it is linked above, and it is the more specific answer of the two.
Lead Appliance Repair Technician · 12 years experience
Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
