if its a non frost free model it should have a mechanical thermostat with capillary with either 2 or 3 terminals,im sure your thermostat is ok just check the end of the capillary is making good contact with the back plate/evaporator.

whats the model number of the fridge freezer?

thermostat is the knob with 1 to 5 on it,set to 1 and check it cycles on and off,if it does i would stick my neck out and say its a tiny amount undergases.probably requires a tiny squirt of r134a/r600.

always difficult to diagnose without looking at it