I had a fridge/freezer which we took out insurance on, just as well, as all through its life it kept having problems, due to the insurance it was a simple case of ringing up and a guy would come out, however we would normally first try a simple defrosting.
The result was when the guy arrived the unit was clean and free of ice, this had gone on for years, by now the unit was 15 to 20 years old, and for once we had not had the time to defrost it, so the repair guy came in, opened the door, and commended whole unit as uneconomical to repair.
He said the insulation had failed, and this had resulted in over time ice would form inside the insulation and as it built up it was transferring heat into areas in an uneven manor, causing some areas to get colder than others, so ice built up in the fridge, and it was also likely what caused the fan to fail in the freezer, seems likely the fault had been there from the day we bought it, and it had slowly got worse over the years, we got compensation, which went towards the new one, and the old one was turned into a brewing fridge, running at 19°C no problem with ice.
As to pocket, the thermostat has a sensor on the end of a thin bit of pipe or mineral insulated cable which looks like a pipe, and in the freezer or fridge there is a larger pipe which this slides into so it is in the right place to measure average temperature, this larger pipe is called a pocket.
I did change on in a small freezer, I plugged the freezer into an energy monitor, and realised it was not switching off, either it had lost gas, which would mean replace, or the thermostat had failed, I tried to find a thermometer to check, but could not find one which could measure -18°C however that's the freezing point of brine, so a saucer of brine froze solid so likely the thermostat. My brewing thermostat arrived STC-1000 so I used that to test, until the correct one arrived.
Today the Inkbird ITC-308 is used, saves the work of making a box etc. I still use my STC-1000 so I can use freezer as a fridge, but with a fridge/freezer this would not work.
Some cheap units just proportion the cooling pipes so fridge is around + 4°C the freezer at - 18°C but most have valves to control the flow in each compartment. Some control motor speed, the inverter drive freezer is claimed to be far better, but it's not my field.
However the control hardware varies a lot, and you would need to look up how your unit works, I looked at mine, and realised without special equipment there was no way I could repair it, I tried to find a fridge man, and to attend would cost £60 before he had done anything, that I think covered a set time, but parts were on top of that, so with no insurance it means once they fail, may as well scrap. That £60 call out is a killer, last thing one wants is to pay £60 to be told the unit is scrap.
