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Ice accumulates heavily on evaporator coils because the thermostat has failed open and never signals the defrost heater to activate, allowing frost to build up into a solid block.
The automatic defrost cycle fails to complete. The bi-metal thermostat is stuck open so the heater never receives the signal to turn on and melt accumulated ice.
Frost-choked evaporator coils block airflow from the freezer into the fresh food compartment, causing the fridge side to warm up even when the compressor runs normally.
When the thermostat sticks closed and doesn't open, the defrost heater runs too long and raises freezer temperatures above safe food-storage levels.
With iced-over coils reducing cooling efficiency, the compressor runs non-stop trying to maintain temperature, leading to excessive energy use and potential compressor wear.
A thermostat that stays closed causes the defrost heater to cycle on and off erratically, resulting in temperature swings that make sections either too cold or unpredictably warm.
💡 Pro Tip: How to Confirm a Bad Defrost Thermostat
Your part is likely the culprit if you notice any of the following:
⚠️ Heavy frost or solid ice block on evaporator coils behind freezer back wall
⚠️ Freezer is not completing its automatic defrost cycle
⚠️ Refrigerator section is too warm while freezer appears normal
⚠️ Compressor runs continuously without cycling off
🔍 Diagnosis Tip: Set your multimeter to continuity mode and probe both thermostat terminals while the part is cold (below 15°F / -9°C). A good thermostat shows continuity when cold and opens (no continuity) once it warms above 40–45°F. No continuity when cold = failed open thermostat. Always test the defrost heater (WP2323198) and defrost control board (4388932) in the same session to rule out other causes before ordering.
📦 What's in the Box
✅ 1× Bimetal Defrost Thermostat (WPW10225581 equivalent, L7.4–15.2C, clips onto evaporator coil)
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Disclaimer: Compatibility information, part numbers, and model fitment data on this page are sourced from manufacturer databases and third-party references. While we strive for accuracy, please verify your appliance model number before ordering. Heima is not responsible for errors or omissions in compatibility listings.
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