When one compartment quits while the other holds, the diagnosis is almost written for you. This page explains dual refrigeration, the frosted coil, the defrost heater and thermistor, and the setpoint-versus-actual check.
Most built-in Sub-Zero units use dual refrigeration: the freezer has its own evaporator and the fresh-food side has another. When only the freezer drifts warm, the fault is in that freezer circuit, usually a frosted evaporator from a failed defrost heater or thermistor, a stalled evaporator fan, or a door seal leaking warm air. The cold fridge is normal because it runs on a separate coil.
The Camino Pablo and Saint Mary's College corridor is full of 1970s and 1980s estate kitchens still running their original 30-plus-year 600-series, IC and IT columns. On units that age, the freezer-side defrost heater and thermistor are the first to go, and a frosted coil chokes airflow long before the compressor is ever the problem. Call (925) 940-3576 for a model-matched check.
The single most useful thing you can do is take two numbers. Read the freezer and the fresh-food compartment with a separate thermometer, then write down what the panel claims for each. A freezer drifting into the teens or low 20s while the fresh-food side sits near 38 degrees is the textbook one-section failure, and it points straight at the freezer's own evaporator, defrost circuit or fan, not the compressor that so many homeowners fear.
Dual refrigeration is why this matters. Because the two compartments are cooled by separate sealed loops, a warm freezer with a healthy fridge is a narrow problem, while two warm compartments is a broad one. The narrow case is usually a frosted evaporator: when the defrost heater or its thermistor fails, frost packs the coil, the fan can no longer push cold air, and the freezer slowly climbs even though the compressor runs. The broad case, both sides warm, sends the technician to the shared condenser first, especially on the old columns common here that heat-soak badly during inland Moraga summers and Diablo-wind spikes.
Key facts:
Measure the freezer and fresh-food temperatures with the same separate thermometer. One warm section with the other holding isolates the failed evaporator circuit and rules the compressor out early.
Write the displayed setpoint beside the measured number. A large gap, panel says 0 while a thermometer says 22, points to a sensor or thermistor rather than the cooling system itself.
Open the freezer and listen. A silent fan, or a visible wall of frost behind the rear panel, suggests a defrost-circuit or fan-motor failure choking airflow.
On a hot or Diablo-wind afternoon, note whether both compartments drift warm together. That pattern moves the first test to the shared condenser and its fan before anything internal.
Capture the model and serial. On an older Camino Pablo column the right defrost heater, thermistor and fan vary by family, and the tag lets the technician stage the correct parts.
| Reading pattern | Likely cause | First test | Planning range |
|---|---|---|---|
| Freezer in teens, fridge ~38 °F | Frosted evaporator from defrost heater or thermistor. | Inspect coil frost, test heater continuity and thermistor. | $440-$1,210 |
| No fan sound, frost wall behind panel | Evaporator fan motor or defrost fault. | Fan motor test and defrost-cycle verification. | $435-$1,045 |
| Both sides warm in a heat spell | Heat-soaked or dusty shared condenser. | Clean coil, confirm condenser fan, re-read after an hour. | $205-$425 |
| Panel reads cold, food thaws | Faulty thermistor or sensor reporting wrong temperature. | Setpoint-vs-actual check, thermistor and board test. | $440-$1,210 |
| Freezer warm only after door use | Magnetic door seal letting warm air in. | Drag test and gasket alignment. | $350-$850 |
These planning ranges are written for Moraga homes and keep the diagnostic branch, the included work, the price and the time window visible before a written estimate is approved.
| Service / branch | What is included | Price range | Time window |
|---|---|---|---|
| Freezer diagnostic | Two-compartment readings, setpoint-vs-actual check, fan and frost inspection. | $125-$195 | 45-90 min |
| Condenser heat-soak recovery | Coil clean, fan verification and a 30-60 min temperature recheck. | $205-$425 | 60-120 min |
| Evaporator fan or airflow repair | Fan motor test, wiring check and post-repair pull-down to 0 °F. | $435-$1,045 | 1-4 hours |
| Defrost heater or thermistor repair | Frost pattern, heater continuity, thermistor resistance and board command check. | $440-$1,210 | 1-4 hours |
| Control board freezer fault | Board output, sensor readings and alarm history matched to the model family. | $640-$1,345 | 2-4 hours |
Final price is driven by which evaporator failed, the unit's age, part availability and cabinet access. A written estimate should name the model, part, labor, price and warranty terms.
Camino Pablo corridor
Estate kitchens still running original IC and IT columns; freezer-side defrost heaters and thermistors are the usual first failure on units this old.
Saint Mary's College area
Older 600-series cabinets that heat-soak during Diablo-wind spells; both-sides-warm calls here start at the shared condenser.
Moraga Hills
Panel-ready built-ins where setpoint-versus-actual gaps catch a failing thermistor before anyone touches the sealed system.
Campolindo
Busy family kitchens with heavy freezer-drawer use; a warm freezer only after door use usually traces to the magnetic seal.
Nearby service context: Orinda, Lafayette, Canyon and Walnut Creek; Moraga ZIPs 94556 / 94570; landmarks include Saint Mary's College, Moraga Commons and the Lafayette-Moraga Trail.
Our 1980s IC-27 column near Camino Pablo had the freezer drifting to 18 degrees while the fridge held a steady 38. Instead of talk about replacing the whole unit, the tech found a frosted-over evaporator from a dead defrost heater, swapped the heater and thermistor, and verified a clean pull-down to 0. The $930 repair matched the defrost branch.
During a hot Diablo-wind stretch our 632 near Saint Mary's College had both sides warming with the compressor running nonstop. I expected the worst. The tech cleaned a heat-soaked condenser, confirmed the fan, and re-read temperatures an hour later to prove recovery. The $295 visit stayed in range and we kept the unit.
The freezer display on our BI-36UFD read 0 but our own thermometer said 22 and the ice cream was soft. The tech explained the setpoint-versus-actual gap, tested the thermistor and board, and replaced the failed freezer thermistor. Two hours, $560, and the real temperature finally matched what the panel claimed.
Call the published number or book online. Have both temperature readings, the displayed setpoints and the model/serial photo ready so the visit can be planned around the failed circuit.
Moraga Sub-Zero Repair is an independent service. We are not affiliated with, authorized by, or endorsed by Sub-Zero Group, Inc.
Most built-in units use dual refrigeration, meaning the freezer has its own evaporator and the fresh-food side has another. When only the freezer is warm, the fault is almost always in that freezer circuit: a frosted evaporator from a failed defrost heater or thermistor, a stuck evaporator fan, or a door seal leaking warm air. The cold fridge is normal because it runs on a separate coil.
When both compartments warm together during inland heat or a Diablo-wind spike, the first suspect is the shared condenser. Original 30-plus-year units in the Camino Pablo and Saint Mary's College corridor run condensers that heat-soak and cannot reject heat fast enough. A clean coil and verified fan should come before any sealed-system language.
A gap between the displayed setpoint and the real temperature usually means a sensor or thermistor is reading wrong, so the control thinks the freezer is cold when it is not. Put a separate thermometer inside, record both numbers and report the difference. The technician tests the thermistor and board against the actual reading before replacing parts.
A healthy freezer holds near 0 °F. If it is drifting into the teens or low 20s while the fresh-food side stays close to 38 °F, that one-section pattern points to the freezer evaporator, defrost or fan rather than the compressor. Record the reading before any reset so the evidence is not erased.
Have the model and serial photo, the freezer and fresh-food readings in Fahrenheit, the displayed setpoints and a note of whether you hear the freezer fan running. For older columns in the Camino Pablo corridor, mention the unit's age so the technician can stage defrost and sensor parts for that family.
