Defrost Heater Sensor Thermostat Kit — Refrigerator Not Defrosting? Here's What to Replace
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Understanding the Problem
When a refrigerator's defrost system fails, frost and ice accumulate on the evaporator coil. That ice layer prevents proper air flow and heat exchange, causing the freezer or fridge to run constantly, lose cooling performance, or produce water from melting ice. The defrost kit (heater, thermostat and sensor/thermistor) is responsible for melting accumulated frost during the defrost cycle and telling the control when the coil is warm enough to stop heating. A failed defrost heater will be open (no continuity) and can't melt the frost. A bad defrost thermostat (or temp cutoff) may not close to allow heater power during cold coil temperatures. A failed sensor/thermistor can report wrong temperatures to the control board and stop the defrost cycle or prevent it from starting. Any one of these parts can cause persistent ice buildup and cooling issues, so diagnosing each component is important before replacing parts.
Common Symptoms
Frost/ice buildup on the evaporator coil, poor cooling in fridge or freezer, long or continuous compressor run times, water pooling under the crisper or in the defrost drain, and occasional warm spots in the freezer.
Common Causes
- Open or broken defrost heater element that can’t melt ice
- Defrost thermostat or temperature cutoff failed (won’t close when cold)
- Defrost sensor/thermistor providing incorrect temperature readings to the control board
Popular Parts That Fix This Problem
These are the most common replacement parts that fix this problem. When you're ready to order, click below to find the right part at ProsourceParts.com — just search by your appliance model number for a guaranteed fit.
Helpful Repair Tip
With the fridge unplugged, remove the evaporator cover and visually inspect for broken heater elements or heavy ice. Use a multimeter to check continuity: the defrost heater should show low ohms (continuity), the defrost thermostat should be closed (continuity) when very cold, and the thermistor should show a temperature-dependent resistance — compare that value to the manufacturer's spec (many thermistors are around 10 kΩ at 25°C).
Frequently Asked Questions
Do I need to replace the entire kit or can I replace only the failed part?
You can replace only the failed component if you correctly diagnose it (use visual inspection and multimeter tests). However, because these parts work together and are relatively inexpensive, many technicians recommend replacing the full kit if there’s significant ice buildup or the appliance is older to avoid repeat failures.
How hard is it to replace the defrost heater, thermostat and sensor? Do I need a technician?
Replacement is a moderate DIY job if you are comfortable unplugging the refrigerator, removing interior panels, and using basic hand tools and a multimeter. Typical steps: unplug the fridge, remove the evaporator cover, disconnect and remove the old heater/thermostat/sensor, install new parts, reassemble and test. If you’re not comfortable working with electrical components or diagnosing the control board, hire a qualified appliance technician.
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Whether your dishwasher won't drain, your dryer stopped heating, or your fridge isn't cold, we've created simple repair guides for the most common appliance problems homeowners face. Each guide explains what's going wrong, the most likely causes, and which replacement parts fix the issue. When you're ready to order, we link directly to ProsourceParts.com where you can search by model number and get the right part shipped fast.