Most models builts since 1994 use 2 actual electronic sensors. Basically the pcboard has to "see" what's happening. It does this through it's sensors.
First, the "Temperature" sensor. This sensor does just what it's name implies. On Jacuzzi Whirlpool Bath hot tubs the temp sensor resides in the filter box. On models built since 2002 by Jacuzzi Premium the sensor is mounted on the wall of the hot tub usually in the filter cabinet.
As water temperature goes up or down it changes the resistance in the sensor. The pcboard senses that and sees it as the temperature and reacts accordingly.
The other electronic sensor is the "High Limit" or OH sensor. It too does what it sounds like. It design to tell the pcboard whether the water is getting to HOT and if so, shut the hot tub down. There are 2 general configurations for OH sensor. Most have the sensor mounted on a clip or tied to the heater. The other style has the sensor screwed directly into the heater housing. Either one mounts on the heater because that will be the first place for the heat to come from and so it's a protection to avoid either allowing the water to get too hot or even causing a fire.
If you want to test to see if one of these sensors has failed you can. You must have a good digital volt/ohm meter. Set to ohms and compare the resistance of the other sensor. They should be very close.
Above is the general resistance values of models built 1994-2002.
Note that when water temp goes UP, resistance goes DOWN.
Above are the general values of models built 2002-Present.