Ammonia smell in freezer

When you smell ammonia the cooling unit is leaking. There is no way to repair it. If you check in back of the fridge, you may see some greenish colored residue.  This is evidence of leaking. Look for a new cooling unit or a new fridge.

Accurately measuring refrigerator temperature

To get an accurate reading of temps, leave a thermometer in a glass of water and read the temps. If you place a thermometer on a shelf, you are only reading air temps, and that is not an accurate indicator of fridge performance.
 

Norcold Refrigerator

Here is a link defining Norcold 3 Year Ltd Warranty
If you are close to the 2 year mark you might get them to include the labor and especially if you have documented your prior problems.

Problem with Norcold Frig

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.

Norcold thermistors on Norcold models up to, but not including the 900 series model,  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.