Leave it upright for 24 hours then turn it on.
it could be that the -21 is the aiming temperature and not the actual temperature - like when you set an oven to do baking
Remember too that European Fridges get set to a centigrade figure -18 C and about +5C
In the USA it's Fahrenheit 0 degrees F and 35 F
Unlikely to be a sensor fault because both fridge and freezer are warm.
But I'm just guessing
Good Luck...
August 2015