Your refrigerator has a defrost thermostat to regulate the fridge temperature. The refrigerator goes through cycles of cooling and rest, which prevents the build-up of ice inside the freezer unit and keeps food in the fridge from freezing. A defective thermostat may overheat as well, which can cause a dangerous fire and ruin the rest of the unit. You should test defrost timers before replacement, using a multitester which is a handheld unit for testing circuits. Multitesters are available from most hardware or electronic stores.
Unplug the refrigerator.
Remove all food and shelves from the unit.
Locate the defrost thermostat compartment in the freezer. It is either located towards the right, near the back of the unit. Remove the screws holding the cover in place using a screwdriver or pinch the tabs to release it.
Pull the wires out from the defrost thermostat using a pair of pliers. Hold the covers of the wires with the pliers and pull. Do not pull on the wires, since this may break them. Look at the wires for any corrosion.
Set your multitester to the X1 setting and touch the probes of the device to the two terminals you removed the wires from. The reading should be zero when the thermostat is cold or infinity when the thermostat is warm. If you get any other read, you should replace the thermostat.
Remove the thermostat by taking off the control knob then removing the screw holding the thermostat in place. Place the new thermostat in place and tighten down the screw. Place the knob on the thermostat. Insert the wires you took off earlier in the same place. Plug in the fridge.