The temperature sensor can be tested with a ohmmeter to determine when/if the switch closes.
If your sensor has (1) electrical terminal, connect one wire of the ohmmeter to that terminal and the other wire to the body of the sensor.
When the water temperature reaches 199* the internal switch will close and the ohmmeter will indicate a complete circuit.
If the sensor is bad it will not switch.
If the sensor switch does close and and indicate a complete circuit remove the heat from the test and observe the reduced temperature when the switch opens. I could be a much as 10* under the closing temperature.
Repeat the test to look for intermittent operation.