Newly designed and includes o-ring. This high-limit thermistor sensor is a safety device designed to communicate with the system to shut down the heater if the water temperature exiting your heater exceeds 118 degrees F. It simply screws into the No Fault Heater Housing (hand tight only) and then plugs into the outside of the spa’s control box. If your existing thermistor plugs directly onto the main circuit board (motherboard) inside the IQ2020 control box, then use item 79005.