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 39204. Always replace thermistors in pairs (ie; 38415 & 38416).
Note: Symptom of this part failing is a flashing red “Power” light on the Hot Spring & Tiger River control panel or on the control box of the 1995-1996 Tiger Rivers. This thermistor is smaller in diameter than the Control Thermostat Thermistor (38415). It screws into the port (hand tight only) marked “outlet” on the PDR replacement heater. Typically, when one thermistor needs replacing, the other is close to failing as well and should be replaced at that same time. Thermistors have a reliable lifespan of 6-8 years or so.
Note#2: ALWAYS REPLACE THE THERMISTORS IN PAIRS. If a new thermistor differs in temperature sensing more than five degrees from the old thermistor, the system will go into a “fault mode” and prevent the heater, jet pump(s) and light from operating. So again, when replacing a control thermostat thermistor always replace the high limit thermistor as well and vice versa.
1. Set the meter to 20K on the OHM scale.
2. Record the temperature of the spa water.
3. Unplug the thermistor from the control box.
4. Meter across the two Thermistor leads.
5. Refer to the Temperature versus Resistance chart below to find the correct resistance value for the temperature measured. If the temperature and resistance do not match that of the chart, this test has failed and the thermistor must be replaced. Remember if the spa has been inactive, the spa water temperature may be different
than the water temperature in the heater housing.
Note: A high-limit fault will kill power to the heater, jet pump(s) and light but the circ pump will remain operational (except on a 1995 Tiger River where the circ pump will also be shut down).