The defrost thermostat on your fridge is an important piece of equipment which prevents your freezer from becoming too cold. However, from time to time, these devices can apparently fail, and cause problems which include leaks upon the floor, a warm refrigerator portion, or a freezer section which has ice forming across the back. If any of these problems occur, you should check the defrost thermostat first, in order to rule this in or out as the cause of the problem. Troubleshooting a defrost thermostat is probably a job for the professional, but if you wish to have a go, then you can, assuming that you have some basic knowledge of how fridges work, and can follow some simple ideas to testing your freezer.
WARNING: Some thermostat components contain toxic substances, so think carefully about whether you want to troubleshoot it yourself.
If Your Freezer Section is Icy
If this is your problem, then you should immediately place the defrost timer onto manual defrost. This should automatically trigger a warming of the freezer section. However, if after 20 minutes the freezer still has not begun to defrost, then you will need to remove the back wall, and look at the thermostat itself. This should be stuck to the evaporator coil.
Bypass the Thermostat
Turn off your power, and then remove the wiring from the thermostat, and connect them to your defrost heater. This should be easily managed when you are looking at the thermostat, the wires will be evident. Turn the freezer back on, and wait for 10 minutes. After this time, if the freezer has begun to melt, then you will need to check the thermostat, as it is definitely the cause.
You should check the voltage continuity of your thermostat, using a multimeter. Connect your probes to the thermostat, and turn it on. When the thermostat has a consistent reading, you should see a Ohms setting of 0. Anything more than that, and the thermostat is defective.
Inspect the Connections
Once the thermostat has failed the Ohms test, then you should check the connections at the back. Pry off the connectors, using a pair of thin pliers if necessary, and remove the wires. You can check the connections are not corroded, and also pass some paper along the surface of the connectors, as this will clear away any dirt. While the wires have been removed, you can also examine them for signs of damage.
Check the Temp
Open up the thermostat, and find the small vial of mercury that measures the temperature. This can sometimes come out of alignment, and will need to be re-set. Using your pliers, turn the nut on the mercury screw until the connection pops out. When this is done, leave for around 10 minutes, and then turn the nut in the opposite direction, until the connection closes again. Your mercury vial should now be reset.
WARNING: Mercury is toxic when inhaled or ingested, so proceed with caution before handling it.
Returning the Thermostat
Replace the thermostat in the freezer, and test the defrost timer again. If the freezer still does not start to defrost, you may have to replace the thermostat completely.