The thermistor is the temp sensor that the thermostat uses to control cooling cycles
The thermistor plugs into the UPPER control/display board...runs over the top of fridge, down backside and then enters food compartment where it is attached to a fin with a small clip.
The thermistor MUST be in contact with the FIN
If not...it will cause over-cooling due to lack of temp sensing.
If thermistor IS attached to FIN then it is no longer functioning and that causes over-cooling regardless of thermostat slider position.
Thermistor part number (same thermistor for all 60/6000 Series)
Norcold 61693822
Unfortunately it is no longer available
So best option is 'Snip-The-Tip' sensor replacement
Snip-The-Tip Replacement
600/6000 Series ManualPARTS