Servis FF M7554/2FF

Got a call to this fridge freezer saying that fridge is warm and freezer ok.
When i got there the fridge had cooled back down and everything looked and checked out ok, ie evaporator clear, fan working fine, reed switch on door seems to be operating reliably. Got call 2 days later to say its doing it again. Now im thinking it can only be one of 2 things, either the fan is breaking down or the thermister in the fridge is playing up. I take it the sensor in the fridge turns the evaporator fan on and off to control the fridge temp as there is no baffle in vent.

Any Help Please?

Thanks

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