Each spa is equipped with a high limit thermistor that is used as temperature sensor. The thermistor is a semiconductor that changes resistance with temperature changes (as the temperature goes up, the resistance goes down). The high limit thermistor trips if the water temperature exceeds 118 degrees F.
The thermistor must be tested prior to diagnosing the electronic high limit circuit board.
NOTE: BYPASSING A THERMISTOR OR SHORTING THE TERMINALS WILL CAUSE THE SPA HEATER TO SHUT DOWN. NEVER ATTEMPT TO BYPASS OR ELIMINATE A THERMISTOR FROM THE CIRCUIT.