This control thermostat thermistor sensor detects the spa water temperature and relays that information back to the system. It simply screws into the No-Fault Heater Housing (hand tight only) and 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 39205. Always replace thermistors in pairs ( ie; 38415 & 38416 ). Check your serial number in the parts finder above to be sure this part is for your spa.
Note: Symptom of this part failing is fluctuating spa temperature even though the temperature setting has not changed. A symptom of this part failing completely is a flashing green “Ready” light on the control panel of a Hot Spring spa model. This thermistor is larger in diameter than the High-Limit Thermistor (38416). It screws into the port (hand tight only) marked “inlet” on the PDR replacement heater. Typically, when one thermistor needs replacing the other is close to failing 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.