Most thermostats that you see in a fridge, freezer or fridge freezers are electro-mechanical in operation although, in recent years with the advent of electronic controls creeping slowly into the domestic refrigeration market, electronic sensors known as thermistors have become more commonplace.
The older method remains popular, especially so on many low cost refrigeration units simply because they are generally cheap, simple and usually fairly robust. They are therefore extremely common.
Diagnosing a problem with a simple thermostat is not that hard, even for the inexperienced as, although many people think that these simple components are more complex than they really are, they are merely a simple on and off switch regulated by temperature.
How Thermostats Work
There is more in this article on how fridge and freezer thermostats work but, the short version is that in the phial as shown in the image below, there is a gas held.
The phial is normally attached to the back wall of the fridge or fridge section in a fridge freezer directly onto or where the evaporator (the bit that cools) is mounted on the wall or behind it.
The gas in the phial then expands and contracts causing the switch in the thermostat body to be operated switching the unit on and off.
This is known as cycling because the thermostat "cycles" on and off in a temperature range.
It should be noted that temperatures can vary here and that it is perfectly acceptable, on a domestic unit at least that isn't being used for special purposes such as medical use, to be something like +2˚C to +7˚C giving a mean of about +5˚C and the deviation can be greater than that at times. The trick here is to remember that this is about the mean temperature, not the extremes.
Inside the thermostat body will normally look something like this:
In our experience on virtually all domestic refrigeration you will see the first kind, the toggle type.
As you can see though, from a servicing point of view this is a very simple device.
Known Thermostat Faults
Almost all electro-mechanical thermostats have two states. On and off.
It is extremely rare to see anything other than that in terms of status in a domestic refrigerator, freezer or fridge freezer. Electronically controlled models are of course completely different and different principles apply but for the older types, dead simple stuff really.
So when a thermostat goes faulty you will either get the unit running all the time or, not at all.
Here's the easy part, if the unit is running all the time and your fridge or, fridge section of a fridge freezer is freezing everything in it then the odds are it is merely a simple thermostat fault.
Replacing the thermostat will cure that fault in almost all cases.
There are other faults that can cause this or, symptoms like this but they are far rarer than a stat fault.
If however the refrigerator or freezer is not running at all then there are a number of other things that can fail and cause that to happen, you can't guarantee that the thermostat is faulty. This isn't so common on fridge and fridge freezer thermostats as usually when they fail they tend to fail closed, so the machine runs constantly however it can and does happen that the thermostat fails open and the machine appears dead.
Temperature Ranges And Cheap Thermostats
There are some sources on the internet that say you can fit any old thermostat to almost any fridge, freezer or fridge freezer. These would usually be the sites that are selling the bog standard universal thermostats for a few pounds, often the cheapest ones you can get.
It isn't true.
As with most things, if it seems to be too good to be true, it probably is.
A lot of thermostats are designed with particular phial lengths or temperature ranges depending on where they are placed within the cabinet and fitting the wrong one can lead to disastrous results.
Your fridge may not cut in at all and not cool or, it may never cut out and freeze everything.
Getting a correct suitable replacement thermostat is essential and, if you don't know what you're doing or looking at, that's going to be pretty hard. So, for a novice, think, impossible.
A lot of the really cheap fridge and freezer thermostats we see smack us with "cheap and nasty" as soon as we see them. We don't sell those, they're cheap and nasty with the emphasis firmly on "nasty".
Do keep in mind that you are relying on this little device to keep your food fresh and safe so that you don't get a dose of e-coli, salmonella, listeria and other nasty stuff.
According to research a fridge salad drawer contains 750 times the level of bacteria that is considered to be safe, you don't want to be adding to it by trying to save a couple of pounds on a replacement thermostat.
OEM Only Thermostats
There are a number of thermostats for all fridges and fridge freezers more than for freezers that are what we call OEM only, that is to say that a replacement thermostat will only be available from the Original Equipment Manufacturer and there is no alternative.
We see a lot of these on integrated fridges and freezers as well as more specialist models, especially those with multiple temperature zones.
These tend to be more expensive, sometimes a lot more expensive but, the positive is that they will fit easily and operate as they should.
If we advise you can only use the OEM thermostat, we pretty much know that will be your only option and whilst you might be able to bodge something up with a cheap thermostat, it probably won't work properly or fit well, if at all.