1. Iced evaporator coil (defrost system failure) — most common
What it is: Modern refrigerators run a defrost cycle every 8–12 hours to keep the freezer evaporator coil clear. When the defrost heater opens or the defrost sensor (thermistor) drifts, the coil ices solid over a few weeks. Once the ice blocks the fan, the freezer can still hold cold but no air reaches the fresh-food side. Often you'll hear a knocking or scraping from the freezer back wall — the fan blade hitting ice.
Fix: Unplug the refrigerator, pull the freezer rear panel, melt the ice off the coil safely, test the defrost heater and the sensor with the power off, and replace the OEM defrost kit. Don't replace the heater and skip the sensor — they fail together. Verify a full defrost cycle ends with the coil clear.
Typical all-in: $285–$465 all-in
2. Evaporator-fan motor failure
What it is: Evap fan motor bushing wears out, the motor stalls, no cold air moves. Symptom: silent freezer back wall, fridge slowly warms over 12–24 hrs while freezer stays cold (because static cold air still sits in the freezer compartment).
Fix: Confirm the motor is not turning with the fan blade free and the coil clear, then swap in an OEM motor + blade kit. Inspect defrost system as the secondary check — a long ice load is what kills these bushings.
Typical all-in: $245–$385 all-in
3. Damper / inlet shutter stuck closed
What it is: On French-door and some side-by-side models a motorized damper controls how much cold air enters the fresh-food side. When it sticks (ice debris) or fails electrically (motor or control), no air enters even though the fan is moving it.
Fix: Service-mode damper cycle test, OEM damper assembly replacement. Verify open-close before reassembly.
Typical all-in: $245–$425 all-in
4. Main control / adaptive-defrost board
What it is: If the main board never commands defrost, the cycle never runs and ice builds even with a perfectly good heater and sensor. Whirlpool and GE main boards are the usual suspects. Usually after a surge or at year 7–10.
Fix: Confirm with a defrost-cycle test (no command = board), swap OEM main / adaptive-defrost board, recommend surge protection on the dedicated circuit.
Typical all-in: $345–$525 all-in