Bosch fridge freezer KKU3200 not maintaining temperature

I have a Bosch KKU3200 fridge freezer with electronic control panel at the top.

It normally runs at the 'eco' settings of -18c for the freezer and +2c for the fridge but recently the freezer temperature has been rising to -12c then cooling down to -18c again.

It's almost silent when the motor and compressor are running, as if nothing is wrong. It seemed happy enough on boost to go to -23c before I reset it.

I assumed it would be a simple matter of Googling for a freezer thermostat but I'm only finding defrost modules and replacement PCBs. Is the defrost module a fancy name for the freezer thermostat? Do I need to check for something else?

Since it was cheap enough at £17.91 with delivery, I've taken a chance and ordered the defrost module on the assumption that it might be requesting heat/defrost cycles unnecessarily. Where is this module located?

The modual, presuming thta really is the thermostat will be loccated in one of the sde panels, in the freezer side.
Usually at the back, but you know how they always place things in stupid places.

Youmay have to pull the outer panels off to find it

I just removed the freezer drawers and the plastic panel behind them. There's a push-fit module at the bottom right of the evaporator which is the same size and shape as the replacement part but I'm only guessing that this is the defrost module. It's covered in a moulded rubber shroud.

The replacement part has two small terminals but the original appears to have its two wires bonded into the module. I could cut the wires, fit a couple of terminals and cover those and the module body with amalgamating tape but this seems like more of a bodge than a standard repair so makes me wonder if I'm even looking at the correct module or if so whether I should be sourcing a replacement with bonded wires inside a moulded rubber shroud just like the original.

I cut the wires from the old sensor, fitted 2.8mm terminals to them and connected them to the new sensor. There was no activity from the compressor so I presumed that I'd fitted the wrong part and refitted the old on, soldering the wires and covering with self-amalgamating tape.

Still no sign of life from the compressor until several hours later when it sprang into life and the freezer resumed its continuous cycle of defrosting and freezing over a ten degree range. My next guess was that the problem had to be caused by an electronic control, so thinking that the display panel PCB also hosted the relevant ICs I bought one of those.

When fitting I saw a much bigger PCB behind it. I couldn't return the part at this stage so decided to fit it. At first it didn't seem to work any better than the old one and I began sourcing the bigger PCB. The same new Bosch part was offered by various suppliers from £270 to £108. I was on the verge of ordering one when I noticed that the freezer had maintained -18c overnight and has now done so for the last few days.

The only remaining problem which I can live with but would prefer to fix is that periodically there's a loud banging sound from the freezer, accompanied by a 1200W rise in power consumption over a period of around six seconds. I'm guessing that this is most likely caused by the defrost heater or possibly the compressor motor not turning when it should, although it sounds fine when it does.

I have been working away and have just returned here.
So I know this post is old.

On further looking for your fridge, I noticed they were first introduced in 1993. Is this correct?
Thts very old for a fridge.
The compressors generally have long gone after 21 years. and they lose their compression strength.

I took a gamble on a new PCB and fitted that yesterday. So far the odd noises have gone and the temperatures are stable