Thought I'd write up a repair I did last weekend. This is a frost-free fridge freezer, single compressor, bought 1997 IIRC. Symptom is that the fridge stopped being cooled, intermittently at first, and then permanently.
First, I had to work out how it worked. The evaporator (inside cold element) is in the freezer only, behind a panel, with a fan to blow air around the freezer compartment. The evaporator has a heating element intertwined to run defrost cycles on the evaporator. There are two air pipes from behind the freezer panel up to the fridge compartment. They form a flow and return air path to the fridge, and there's a motorised flap on one of these to open/close it.
On the web, failure of this design to cool the fridge is not uncommon, the two reasons typically being the fan motor burns out, or the air pipe gets blocked with ice. Neither applied in this case.
A few weeks ago I had a go at fixing it, but I had almost no tools with me at the time. Having established that the fan was not burned out, and that there was no ice in the freezer, my suspicion was a thermister in series with the fan. I shorted it out, and the fridge started working. However, within days, it became intermittent again, so that wasn't the problem.
At this point, armed with soldering iron and some resistors/LEDs, I added some lights to show when the microcontroller was driving the compressor, the fan, and some other things. It became obvious that the fan output from the microprocessor was not operating most of the time I expected it to. Indeed, it was mostly off, but flickered occasionally.
Without the fan, it seems there is just enough convection cooling to keep the freezer cold, but nothing to drive cold air into the fridge. As a temporary measure, I disconnected the fan output from the microcontroller, and jumpered it across to the compressor output signal. I wasn't really expecting this to work too well, as that would mean the fridge would only cool when the freezer was calling for the compressor, but thought it might keep the family going a few days until they can get a replacement.
However, much to my surprise, it seems to be working perfectly like this. both fridge and freezer are keeping rock stable temperature. It may be that at some extreme of ambient temperature, the fridge could drift from its setting - we'll see.