Building: Pre-war co-op, Upper West Side
Unit: Sub-Zero BI-36S, installed 2007
Symptom: Refrigerator section warm (52°F), freezer still cold
Result: Diagnosed defrost system failure. Repaired same-day. Total cost: under [on-site quote].
The call
11:47am Tuesday. Owner called from her Upper West Side co-op — Sub-Zero BI-36S running but the refrigerator section had been getting warmer over 48 hours. Milk turning, vegetables wilting. Freezer "still seems cold." Hadn't called anyone yet.
This is a classic warning pattern: when the refrigerator runs warm but the freezer is fine, the problem is rarely the compressor (the compressor serves both). It's almost always defrost system or a fan motor isolated to the refrigerator side.
The diagnosis
We arrived at 2:30pm. First thing — infrared thermometer reading inside the refrigerator: 52.3°F. Freezer: 4°F (correct). That ruled out compressor, sealed system, and most thermistor failures (those would affect both compartments).
Pulled the back panel of the refrigerator section. The evaporator coils were completely iced over — frost buildup so heavy it had blocked airflow through the coil. The cold air the freezer was making couldn't reach the refrigerator section because the air pathway was iced shut.
That points to defrost system: the defrost heater that's supposed to clear ice off the evaporator at scheduled intervals had stopped working. Three possibilities: defrost heater itself, defrost thermostat, or the control board's defrost timer.
The fix
Tested the defrost heater with a meter: open circuit. Heater had failed. Sub-Zero defrost heaters are a stocked part on our truck — replaced in 35 minutes. Verified the defrost thermostat was still functional. Verified the control board's defrost cycle was running on schedule by triggering it manually.
Cleared the ice buildup from the evaporator (warm water bottles + patience — about 25 minutes to fully clear). Reassembled. Ran a diagnostic cycle to confirm everything was working. Refrigerator started cooling within 20 minutes; reached 38°F within 4 hours per follow-up.
The bill
- $125 service call (credited toward repair)
- Defrost heater (OEM): quoted on-site
- Labor: quoted on-site
- Total: quoted on-site
180-day warranty on parts and labor. The customer's food in the freezer was fine; refrigerator inventory was the loss (about [on-site quote] of vegetables and dairy she'd already discarded before calling).
What we want you to take from this
When a Sub-Zero runs warm in the refrigerator but cold in the freezer, it's a defrost issue 90% of the time. Don't panic about "needing a new compressor." The repair is typically under [on-site quote] and same-day. The longer you wait, the more food you lose, but the underlying unit isn't damaged by short-term warm operation. Call us.
Frequently Asked Questions
How long do Sub-Zero defrost heaters typically last?
Most defrost heaters last 10-20 years. A 2007 unit having a heater failure in 2025 is right in the expected range.
Could I have prevented this?
Annual maintenance plan visits catch defrost system early warning signs (slight frost on evaporator before it becomes major buildup). The $250/year plan would have caught this 6-12 months earlier when the cost might have been a fraction of the cost.
Will this happen again?
New defrost heater under warranty for 180 days from us. The other defrost components were tested OK during the visit. Realistically, another component could need attention 5-10 years from now — that's normal aging.