HydroQuip ECO-1 Circuit Board spa controls use this hi limit temperature sensor 34-0203A with a 120"" long (10 ft) 3-wire cable with a 4-pin plug, and a 3/8"" x 1.5"" long stainless steel sensor bulb. Temp Sensor WH-3 is used on 6100/7100 Series spa controllers. Also used on Gecko MSPA/TSPA controls to replace Gecko 9920-400125. The bulb goes into the thermowell and the plug connects on the pc board.
Signs of temp sensor failure include different display and actual temperatures, or HL error codes. Check the plug and wiring for any damage, or other obvious signs of failure, which may not always be evident.