Sensor Tutor:

Jacuzzi Hot Tub's use 2 actual sensors and a third sensor more commonly called the pressure switch or flow switch. 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 most Jacuzzi Hot Tubs the temperature sensor is in the filter area. It looks like a little gray plug.  As temperature goes up or down it increases or lowers the resistance in the sensor. The pcboard senses that and sees it as the temperature and reacts accordingly. On these spas the temperature sensor also acts as a backup High Limit or Overheat sensor. So if the spa goes above 112-115 degrees it will see that also and shut off.

The other electronic sensor is the "High Limit" or Overheat 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 spa down. There are several configurations of the high limit switch. It is either a sensor clipped against the heater or a sensor that touches the heater.  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 out can. You must have a good digital volt/ohm meter. Set to ohms and compare the resistance of the temp sensor and the OH sensor. They should be very close. On a very few models the OH sensor is a stud with 2 wires connected to it. It simply is an on/off sensor. If the water goes above 112-115, the circuit opens and shuts down the spa. Most Jacuzzi Hot Tubs use sensors using resistance and reporting that to the board.