Optical Sensor & Thermistor that gives Water & Prime errors on SpaNET and Vortex spa control systems. This is a common point for leaks. Often due to low pH or other water issues. ER-2, ER-3, ER-6.

SV Series: (square plug)
Suits SV2, SV3, SV4
XS Series: (round plug)
Suits XS-2000 | XS-3000 | XS-4000 Optical Sensor and Thermistor Assembly. Comes with Round plug. 

These items cannot be returned once fitted. We recommend replacing entire heater where these have leaked.