The way I like to test these thermistors is to place the thermistor in a cup of ice with a small amount of water. What this does is it gives you a very controlled test. Inside the cup of ice with water the temperature should be very close to 32 degrees. This way you know for sure what the temperature is and at or very close to 32 degrees the thermistor will be about 16.3K ohms. This can be done without removing the thermistor in most cases (it is a little hard on the freezer thermistor on side-by-side models) simply sit the cup on a nearby shelf. Make sure you leave it in the cup for at least 5 minutes. Readings are +/- 5%. The good part is that most of the time if a thermistor is bad it will be way out of range.
Then you can make your test from the J1 connector on the main board.
For the thermistor you need to be checking you test from pin 5 to pin 4 with the connector disconnected. This is the evaporator thermistor and it controls the defrost cycle.
Have you looked at the evaporator coils to see if they are frosted up indicating a defrost problem? If so you need to test defrost heater and thermostat and if they are ok then the only thing left is the evaporator thermistor. If it is not a defrost problem then the fresh food thermistor may be bad or the damper may be bad. You also need to check the air ducts and make sure they are not clogged with ice.
This pins for the fresh food thermistor is pin 5 to pin 1
These two pages will also helphttp://www.appliance-repair-it.com/refrigerator-thermistor.htmlhttp://www.appliance-repair-it.com/GE-refrigerator-repair.html