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How to Test HotSpring Spa Temp Sensors

Thermistor, High-Limit, Hot Spring, Tiger River, Limelight

In this post, we are going to teach you how to test the Temp Sensors, aka Thermistor, in your HotSpring Spa. Over the years, HotSprings has used a few different types of temperature sensors in its control system. The procedure for testing them is identical.

Spa Temp Sensor Function

Thermistors (Temp Sensors) are safety devices designed to communicate with the system to shut down the heater if the water temperature exiting your heater exceeds 118 degrees F.

 They simply screw into the No-Fault Heater Housing (hand tight only) and then plug directly onto the motherboard inside the IQ 2020 control box. If you have an older spa, your thermistors will most likely plug into the side of the control box. If you have a Solana or HotSpot spa your thermistor set up will a little bit different. 

Note: Always replace 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. If you are replacing a control thermostat thermistor always replace the high limit thermistor as well and vice versa.

Testing Your Spa Temp Sensors

What you will need:

Standard Phillips Screwdriver

A Multi-Meter (Ohm Meter)

The first step is accessing the spa’s control box. Find the main compartment door and use the Phillips screwdriver to remove the 4 screws holding the door in place. 

For Spas Built Before 2002, 2nd quarter: Unplug both thermistors from the side of the control box.

For Spas Built After 2002, 2nd Quarter: Use the Phillips Screwdriver to remove the lid of the IQ2020 control box. Then, unplug the thermistors from the motherboard. 

 Test Procedure

  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.
Sensor resistance v temperature chart-

 If the temperature and resistance do not match that of the chart, this test has failed and the thermistors must be replaced.

You can purchase new replacement thermistors here:

Old Style (plugs into the side of the control box):

Control –

Hi-Limit –

New Style (plugs directly into motherboard):

Control –

Hi-Limit –

 Remember if the spa has been inactive, the spa water temperature may be different than the water temperature in the heat

Our goal here at Backyard Plus is to help save you money by avoiding costly service calls for repairs that you can easily do yourself!

If you are looking for HotSpring Spa replacement parts or water care products for your spa, we have what you need!

For more helpful tips and How-To Videos, visit our Video Page!

Thanks for reading!

If there are any topics you would like to learn about pertaining to your HotSpring Spa, please let us know.