The defrost thermostat of your fridge automatically defrosts your freezer. It also keeps your refrigerator running at a consistent temperature to prevent overheating of the unit, which can cause leaks, food spoilage and potentially a breakdown of the condenser unit. The thermostat is a small coil that when wound, makes a circuit. As it warms in the thermostat, the circle expands, breaking the circuit and cutting off the condenser. If the circuit is stuck in the closed or open position, you'll have issues.
Unplug the refrigerator and open up the freezer to defrost it.
Remove the screws from the floor of the freezer, or remove the clips that hold it in place.
Pull off the wires of the thermostat. They are usually attached with slip-on connectors. Pull on the connector with a pair of pliers if stuck. Do not pull on the wire since this may cause it to break. Replace the thermostat if the connectors look worn or corroded.
Test the thermostat for continuity using a mutlitester, a device used to determine whether an electric circuit has continuity. Set the multitester to the X1 setting. Place the two probes on the two terminals of the thermostat. The thermostat should let off a zero reading when cold and an infinity reading when warm. Replace the thermostat if it does not have this reading.