The thermistor is a thermal resistor that changes resistance depending on its temperature. It clips onto the fins inside the refrigerator and communicates temperature changes to the appropriate circuit board by the change in its resistance. Normally when a thermistor goes bad it has a very high resistance and causes over cooling.

Dometic Thermistor
To test the Dometic thermistor, disconnect it from the main (lower) circuit board and place the other end (sensor end) into a glass of ice water. After a few minutes you should be able to get a resistance reading of 7,000 to 10,000 ohms between the two terminals of the connector. If the resistance isn't close to those numbers, it is bad.

Norcold Thermistor
On Norcold models up to, but not including the 900 series model, the thermistor should have a reading of around 40,000 ohms between the terminals of the connector when the sensor end is place into a glass of ice water.