Quick answer: To test a fridge defrost thermostat, unplug the fridge, remove the thermostat from the evaporator coil and check it by sight. Then use a multimeter on continuity. At room temperature it normally reads open. After chilling it in the freezer it should read close to zero ohms. If it does not change, replace it.
What a defrost thermostat does
A frost-free fridge melts the frost off its evaporator coil with a heater. The defrost thermostat is a small temperature switch clipped to the coil. It is wired in series with the heater. While the coil is cold it lets power reach the heater, and when the coil warms up it opens and ends the defrost cycle.
If the thermostat fails open, the heater never runs and frost builds up. If it fails closed, the heater can keep running longer than it should.
Signs the defrost thermostat may have failed
- Thick frost or ice on the freezer back wall that keeps returning.
- The freezer is cold but the fridge section is warm.
- The heater tests fine but the coil still ices up.
- The thermostat looks bulged, cracked or discoloured.
The same signs point to the heater and timer. See our guides on a freezer not defrosting, testing a defrost heater and testing a defrost timer.
Before you start
- Switch off and unplug the fridge. Do not measure on a live circuit.
- Let the fridge thaw if the coil is iced. You need access to the part.
- Take a photo of the wiring before you disconnect anything.
- Find your wiring diagram if the fridge has one, and check how the thermostat is wired.
How to test a fridge defrost thermostat, step by step
- Find the thermostat. It is usually clipped to the evaporator coil behind the freezer back panel. Remove the shelves and the panel to reach it.
- Look at it. Replace it if it is bulged, cracked, melted or has damaged leads.
- Disconnect its leads. Disconnect at least one side so you do not read a parallel path through other parts.
- Measure at room temperature. Set your multimeter to continuity or low ohms and touch a probe to each lead. A thermostat that has warmed up normally reads open (no continuity).
- Chill it. Place the thermostat in the freezer for a few hours, then measure again straight away. A cold thermostat should read close to zero ohms.
Result: open when warm and conducting when cold means the thermostat is switching. If it stays open when cold, or stays closed when warm, replace it. Thermostats differ between models, so check the markings on your old part before ordering.
Defrost thermostat, heater or timer: which part has failed?
| Part | What it does | Quick test |
|---|---|---|
| Defrost thermostat | Ends the defrost cycle | Open when warm, conducting when cold |
| Defrost heater | Melts the frost | Continuity with the fridge unplugged. See our heater test guide |
| Defrost timer | Starts the defrost cycle | Contacts switch when advanced by hand. See our timer test guide |
Test all three. A new thermostat will not fix a burnt-out heater, and a new heater will not help if the thermostat never lets it run.
When to replace the thermostat
Replace it if it is damaged, stays open when cold, stays closed when warm, or if you are replacing the heater and the thermostat is old. Match the markings on the old part and your model number before ordering. Fitting should be done by a licensed refrigeration mechanic or electrician because the thermostat is part of a 240V circuit.
Defrost thermostat we stock
Our defrost thermostat, Klixon type 240V (SL5708, L50-25F, RF027M) costs $13.50. It is a universal part, so compare the part number and mounting with your old thermostat. Browse all defrost timers and thermostats and fridge defrost heaters, or see Westinghouse fridge parts.
Not sure what fits? Use Find Your Part and send your model number. Orders placed by 6pm Sydney time ship the same business day from Colebee, NSW.
Defrost thermostat FAQs
What does a defrost thermostat do?
The defrost thermostat is a temperature switch fixed to the evaporator coil. It is wired in series with the defrost heater, so it lets the heater run while the coil is cold and cuts power when the coil warms up, which ends the defrost cycle.
How do I test a defrost thermostat with a multimeter?
Unplug the fridge, remove the thermostat and set the meter to continuity or low ohms. At room temperature it normally reads open. After chilling it in the freezer it should read close to zero ohms. A thermostat that never changes has failed.
Why does the thermostat need to be cold to test?
It is a temperature switch, so its contacts change with temperature. A cold thermostat should conduct. At room temperature it usually does not. Testing only at room temperature cannot show whether it closes when cold.
Where is the defrost thermostat located?
It is usually clipped to the evaporator coil behind the freezer back panel. The location differs between models, so check your manual or model number before removing panels.
Can a bad defrost thermostat cause ice build-up?
Yes. If the contacts stay open, the heater never receives power and frost builds up. If they stay closed, the heater can run too long. Always test the heater and timer as well.
Can I replace a defrost thermostat myself?
It is wired into a 240V defrost circuit, so fitting should be done by a licensed refrigeration mechanic or electrician. Photograph the wiring and unplug the fridge first.