Welcome to Sensor Tutor:

 

Jacuzzi electronic spas use 2 actual sensors and a third sensor more commonly called the pressure 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 all Jacuzzi Whirlpool spas the actual sensor resides in the filter box through a whole in the box. It has a rubber "grommet" that fills the void between the sensor and the filter housing to hold the water back. 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.

The other electronic sensor is the "High Limit" 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 2 configurations of the high limit switch. Most have the sensor mounted on a clip on the back or bottom of the heater housing. The other style has the sensor screw directly into the heater assembly. 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 each. They should be very close.

The 2 generations of sensors mentioned above are easy to ID. 1995-1999 used a pcboard that required the older sensors. With power off, remove the control box cover. You will see 4 wires coming to a plug and paired together. The plug will be red. That is the older generation sensor and system.

In 1999 they introduced a new line that used black plugs instead of red. The black plugs for each sensor are kept separate from each other and plug in separately.

The third sensor, the pressure switch, covered at: Pressure Switch Tutor