The 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 directly onto the motherboard inside the IQ 2020 control box. (If your existing thermistor plugs into the outside of the spa’s control box see item 38416). Please use the “Parts Finder” feature to confirm that this part is compatible with your spa.
Note: 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.