Siemens Fridge Freezer KG44U190Gb/01

Customer called to say alarm was going off for freezer and the temperature was showing -14. It had been doing this on and off for a couple of days. When I got there she had discovered that putting it on fast freeze had got the temp back down to -25. I can only think it must be a sensor problem because while on fast freeze all was ok. Evaporator just lightly frosted as normal, fans in fridge, freezer and at back all ok.
Firstly there doesn’t seem to be a sensor in the fridge. Is it foamed in?
Secondly I’m thinking maybe its just a faulty temperature ntc in the freezer and that it was just getting an incorrect reading and alarm. Once it got down to -25 as long as the sensor was reading -18 the alarm would not go off.

Any Ideas Please

Thanks

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