Electrolux ER78311 Fridge Freezer

Hi guy’s, I have the above fridge freezer which has developed a problem with the fridge. I had de-frosted the unit about a week ago as the back of the fridge was completely frozen up, after de-frosting the fridge appears to be almost warm!!
I looked through the forum and did as some have suggested and leave swithced off for 72hrs, this has not resolved the problem. I have noticed mention of thermistors, could this be a possible fault? What are they and what do they do? The unit is appx 6 years old so not sure if its worth spending money on it to get it looked at or just buy a new one. I have no problem with getting stuck into it if I have to, but don’t want to waste money calling someone out as there is nobody local as I’m in a remote location.

Any help would be much appreciated. Excellent forum by the way.