Pull the fridge out from the wall to expose the compressor unit. Locate the thermostat directly underneath the freezer bottom on the underside of the unit.
Pull on the wire connectors to unplug the thermostat from the unit.
Set a multimeter to the X1 setting. Touch the probes to each of the terminals in the wires. The multimeter should read zero when the thermostat is below 40 degrees, and infinity when the temperature is between 40 and 90 degrees.
Remove the thermostat if you receive any other reading from the multimeter and replace the thermostat.