Diagnosis
Sub-Zero not cooling: the four checks, in order
Most not-cooling calls resolve to one of four things, and they can be separated in about fifteen minutes with a torch, a hand and a bit of patience. Doing them in the wrong order is how people end up paying for a sealed system they did not need.
Nothing here requires a gauge set. All four are observations, and together they narrow the fault to one of three families.
Four observations, taken in this order
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Is the condenser fan spinning?
Pull the lower grille. The fan should be turning any time the compressor is running. If it is not, the box cannot reject heat regardless of what else is healthy, and you have found a fault that is cheap to fix. While the grille is off, look at the fins: grey fur is dust, white chalk is chloride corrosion, and the two have different answers.
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Is the compressor running?
A flat palm on the compressor shell tells you more than listening does: running means warm and faintly humming. Silent and cool with the box warm means it is not being asked to run, or it is trying and dropping out on its overload. Silent and very hot means it has been trying and failing, and that is a start-component or winding question.
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Is the evaporator fan running with the door shut?
Door switches stop the interior fans when the door opens, so you have to defeat the switch to hear it. No fan means no cold air moving to the compartment, even with a perfectly cold evaporator. This is the single most common cause of one warm compartment.
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Is the evaporator frosted, or bare?
Behind the rear panel. Solid frost across the coil means the defrost circuit has stopped clearing it and the airflow is choked. A bare, dry coil while the compressor runs means there is nothing in it to boil off — a lost charge or a restriction. That is the one observation that separates a hundred-dollar day from a sealed-system day.
| Condenser | Compressor | Evaporator | Most likely |
|---|---|---|---|
| Blocked or corroded | Running constantly | Normal frost pattern | Heat rejection. Clean, or replace the coil |
| Clean, fan stopped | Running, cycling on overload | Normal or light | Condenser fan motor |
| Clean, fan running | Running | Solid frost across the coil | Defrost circuit: heater, bimetal or control |
| Clean, fan running | Running | Bare and dry | Sealed system: lost charge or restriction |
| Clean, fan running | Silent and cool | Bare | Control, thermistor or start components |
| Clean, fan running | Silent and hot | Bare | Start relay, overload, or the compressor itself |
| Clean, fan running | Running | Cold coil, warm compartment | Evaporator fan or a blocked air path |
Tell us which row you landed on when you call. Even if you are not sure, describing what you saw at the grille and behind the rear panel changes what goes on the van.
None of this fixes anything. All of it buys time and keeps the diagnosis clean.
What helps, and what does not
- Keep the doors shut. Every opening dumps the cold air and pulls in humid marine air, which then frosts the coil and makes the fault worse.
- Move what matters into a cooler. A full freezer holds temperature far longer than a nearly empty one, so leave the frozen mass where it is if you can.
- Do not keep changing the setpoint. It gives us nothing to work from and it does not make a struggling system work harder.
- Do not run a hairdryer at the frost. Melting it hides the evidence and the box works for a day, which resets the diagnosis to zero.
- Write down the temperature. An actual number in each compartment, taken with any thermometer, is worth more than a description.
Questions we get on this one
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How long can food stay safe while I wait?
A full freezer holds usable temperature far longer than a half-empty one, and every door opening costs hours. Keep it shut, move anything critical to a cooler with ice, and do not refreeze anything that has gone soft in the middle.
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Should I turn the temperature dial colder?
No. If the unit cannot reach its current setpoint, asking for a lower one changes nothing except how long the compressor runs. It also loses you the useful information of what the box was actually holding.
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The compressor is running. Does that mean the sealed system is fine?
It means the compressor has power and is turning. It says nothing about whether there is enough refrigerant for it to move, or whether it is developing pressure. A running compressor with a bare, dry evaporator is the classic signature of a lost charge.
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One side is cold and the other is not. Is the whole unit finished?
No. Sub-Zero built-ins from the 600 series onward run two independent sealed systems, one per compartment. A failed fresh-food side leaves the freezer working normally and is repaired on its own.
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It was fine yesterday. Can something fail that fast?
Fans, relays and boards fail instantly. Condensers do not; they degrade over years and then get exposed by the first warm afternoon. Sudden is usually the day you noticed, not the day it started.
Book it, or talk it through first
Give us the model number and what the box is actually doing. We would rather spend two minutes on the phone than turn up with the wrong part.
$89 service call, waived when you book the repair. Monday to Saturday, 8am to 6pm.