BEKO CF6914W Fridge within Fridge-Freezer is freezing up

Fridge is frozen such that tonic water has become solid.

Compressor runs most of the time.

Switched fridge off for 2 hours and tested the wiring that goes from the fridge temperature control to a control device (thermostat; control board or thermister) reading when off for 2hrs was about 45K Ohms. At this time the rear element was cool.

Switched fridge on. Compressor or freezer fan did not start to begin with. After about 8 minutes checked and found fan running and the rear element was warm. Time 14th May 2018 at 2015hrs.

Check reading on temperature control board again in 2hrs to see if the reading of 45K Ohms changes.

2240hrs reading was in excess of 7M Ohms and declined as I left the fridge door open and monitored. Within 3 minutes the reading was 1M Ohm and still dropping.

Reconnected the fridge temperature control potentiometer which seems to be working fine. Plugged back into mains. Device’s motor started briefly, them stopped and stayed silent from 2252 till 2257. The compressor and freezer fan then started to run.

When the fridge door is opened the motor or freezer fan do not stop when running prior to opening the door.

I am looking to identify the process to remove the faulty parts and replace them. Can you help by advising which components most likely need to be replaced and where they are located and how I get access to them. If I have to remove foam insulation then I would like to know what I replace it with. Would it be a general purpose expanding builders foam? If you have any advice how to test the potential faulty items that would also help.

Many thanks,
Mike