Why is my Sub-Zero refrigerator not cooling?

The reasoning behind that

These built-ins cool the two compartments with two independent sealed systems, and a freezer still holding hard ice takes the costly repair off the table before anyone opens a toolbox. The fault is on the refrigerator circuit: usually its evaporator fan has stopped, the defrost has failed and iced the coil, or the thermistor that reports compartment temperature has drifted. In a Mission Hills kitchen there is a fourth candidate worth checking yourself, and that is the door. Heavy custom panels pull these doors out of alignment over the years, and a gasket that no longer meets the frame will hold a compartment several degrees warm all on its own.

How the diagnosis goes

There is a test you can run before you call. Put a glass of water with a thermometer in it on a middle shelf overnight, then read it first thing in the morning and again around five in the afternoon. If the morning number is acceptable and the afternoon number is not, the unit is losing to heat load rather than failing outright, and the first place we look is the condenser and the air path to it. Kitchens in these houses face west more often than the climate deserves, and many of them have no air conditioning to take the edge off.

If both readings are bad, the failure is inside. We take the interior panel off, look at whether the evaporator is clear or under ice, and check whether the fan turns when it should. A coil under ice is a defrost fault, and the parts involved are the heater and the thermistor. A clear coil with a fan that is not turning is the fan or its control. A clear coil, a turning fan and a compartment that is still warm sends us to the control board and then to the refrigerator's own sealed system, which on this equipment is separate from the freezer's.

If that matches what yours is doing, say so when you call — it decides which parts come on the van.