Compatible With Watkins Spa Thermistor Control Thermostat, Hot Spot & Solana 71578 / HTCP71578. This is the replacement thermistor for the 1999-2002 Hot Spot RE, RLX, SE, SLX and the 2001-2002 SLXtra (wood sided) spa models. Also the Solana 2000-2002 X, 2003-2005 RX, SX and TX models. This thermistor communicates the actual water temperature to the controller to shut off the heater when the spa reaches target temperature.