Things You'll Need
External refrigerator thermostat
Refrigerators contain a built-in thermostat with a control inside the appliance for adjusting the temperature. The wheel or knob control is typically situated inside the refrigerator at the back, while the thermostat is within the appliance cabinet behind the control. If the refrigerator is not cooling and you suspect the thermostat might be defective, a simple way to diagnose the problem is with an external thermostat to bypass the built-in thermostat. If the refrigerator begins cooling after running the bypass, you'll know the built-in thermostat is defective.
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Open the refrigerator door to dial the built-in thermostat control to the coldest setting.
Unplug the refrigerator power cord from the wall outlet and plug in the cord to the socket on the back of the external thermostat.
Route the probe wire on the external thermostat between the refrigerator door gasket and the outer edge of the refrigerator. The probe fits between the gap in the open door and the front of the appliance. The wire is thin and flexible, and will not interfere with the normal opening and closing of the door.
Tape the probe to the interior side wall of the refrigerator as far back as possible with a piece of duct tape. Close the door.
Plug in the external thermostat's power cord to a wall outlet. This bypasses the built-in thermostat.
Dial the external thermostat's control to the desired temperature. For testing purposes, dial the unit as low as it will go. If the refrigerator starts cooling normally, the internal thermostat is defective. If the appliance still does not cool, you have a different problem, possibly involving the compressor motor assembly, the refrigerant gas condenser or a wiring problem.
Running a bypass is a temporary solution unless you wish to use the refrigerator side only because an external thermostat will not maintain freezer temperature correctly to keep foods frozen.
James Clark began his career in 1985. He has written about electronics, appliance repair and outdoor topics for a variety of publications and websites. He has more than four years of experience in appliance and electrical repairs. Clark holds a bachelor's degree in political science.