Help with a hot tub heater testing

it's possible. The red one work with water temperature. In most cases, a NTC sensors are used in such heaters.
Try the following- disconnect the sensor from the PCB and measure it's resistance with multimeter in Ohm meter mode. Add a resistor in series with the sensor, it's value should be equal to a half of the sensor's resistance. To do so, you need to cut one wire to the sensor and solder the resistor in series.
Power up the heater and perform the same test as you made before .