changing a new thermostat on a kelvinator fridge freezer ?

hi
our fridge freezer went. we purchased a thermostat. my hubby fitted it in but the fridge freezer will not switch off and the fridge keeps freezing up. my hubby then purchased another thermostat thinking that the first one was faulty, but again it works but doesnt switch itself off when temperture was reached, and ices up in the fridge.
can anyone help?? not sure if there was a certain way the thermostat is fitted in, or is there another problem.