Most likely causes, in the order we check them
| Cause | What you see | Fix |
|---|---|---|
| Door gasket worn or torn | Humid air is drawn in continuously. Frost forms nearest the seal and the unit runs far more than it should. | Technician |
| Door out of alignment | Heavy custom panels drop over the years. The gasket touches at the top and gaps at the bottom - a new gasket alone will not fix it. | Technician |
| Defrost heater failed | The evaporator ices over solidly, airflow stops and the compartment warms up even though the compressor is running. | Technician |
| Defrost thermistor or control | The cycle never runs, or runs too briefly. Same result as a failed heater, different part. | Technician |
| Blocked defrost drain | Melt water cannot leave, so it refreezes as a sheet on the floor of the compartment - and eventually appears under the unit. | Technician |
| Door left ajar or overfilled | A drawer or a package holding the door open a few millimetres will frost a freezer within a day. | You |
| Coastal humidity and habits | Frequent, long door openings in a humid house put in far more moisture than the defrost cycle was designed to clear. | You |
What you can safely check yourself
- Where the frost is. Frost near the seal points at the door; a solid sheet at the back of the compartment points at the defrost circuit. Tell us which - it changes what we bring.
- The dollar-bill test. Close the door on a bill and pull it out at several points around the seal. Easy slide anywhere means the gasket is not sealing there.
- What is loaded where. A box holding a drawer proud, or an item leaning on the seal, keeps the door open just enough to frost the compartment.
- Ice under the drawers. A sheet on the floor of the freezer is a classic blocked drain, not a cooling problem.
- How often it comes back. If you have cleared frost more than twice this year, the cause is a part, not the weather.
Frost in the freezer versus frost in the refrigerator
Some frost in a freezer is normal and gets cleared automatically several times a day. Frost you can see building week to week is not.
Ice inside the fresh-food compartment is always a fault. It usually means the evaporator behind the rear panel has iced over, so cold air stops circulating and moisture condenses and freezes where it lands.
Why the ice keeps coming back after you clear it
Defrosting the appliance manually removes the ice but leaves whatever let the moisture in or stopped the cycle. If the gasket is tired, it will frost again within days. If the defrost heater has failed, it will frost again as soon as the evaporator has run for a day.
The useful information is how fast it returns. Days points at the door. A week or two points at the defrost circuit. Months points at habits and humidity.
What the visit looks like
We check the seal all round the door, door alignment and closing force, the defrost heater and thermistor, the drain path, and the state of the evaporator behind the panel. Gaskets and defrost parts are common enough to carry, and door alignment is adjusted the same visit.
Related Sub-Zero problems
Questions we get asked
Is a little frost in a Sub-Zero freezer normal?
A light dusting that comes and goes is normal - it is what the automatic defrost cycle is there to clear. Sheets of ice, frost you can scrape into a handful, or ice on the drawer rails are not.
Can I just defrost it myself and carry on?
You can, and it will work for a while. If the ice returns within a few weeks, the cause is a part - most often the door gasket or the defrost circuit - and clearing it again only postpones the visit.
Why is there water under the refrigerator as well as ice inside?
That combination is the signature of a blocked defrost drain. Melt water backs up, freezes inside, and the overflow finds its way to the floor. It should be dealt with quickly - the flooring under a built-in is the expensive part.
Does replacing the gasket fix a door that has dropped?
No. A gasket fitted to a misaligned door will fail again in the same place. The door has to be brought back into alignment first, which is part of the same visit.