The fan in the freezer section should run all the time except when you are in a defrost cycle. Defrost is normally 20 minutes every 8 hours.
Not sure which parts you mean by thermostat and thermometer.
The unit has three parts for defrosting
1 Timer - if it never turns off your defrost timer is shot (motor not working)
If it does turn off and cycle then it is either the defrost timer contacts,
heater is open, defrost thermostat is open at low temp.
2. Heater
3. Defrost thermostat
All must be working for defrost to occur.
Here is a good site with explanations and troubleshooting refrigerator problems and solutions, not cold enough, frost free repair information | 
What Am I Missing? 
I have replaced my thermostat and my thermometer. My coils still keep freezing over making the fan constantly run. I know the coils work when I defrost and reconnect. Am I wiring wrong. I am about to chunk this POS. One last shot if you've got one. Thanks!
