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How to Test Hot Spring Spa Temp Sensors: 5 Simple Instructions

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In this post, you`ll learn how to test the spa temp sensors, aka Thermistors, 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 these spa temp sensors is identical to the procedure with previous sensors.

Spa Temp Sensor Function

Thermistors (Spa 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)b

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 for Spa Temp Sensors

  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.
how to test hot spring spa temp sensors

 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 heater.

We hope this article on how to test spa temp sensors was helpful!

Backyard Plus is Here For You.

Questions regarding your spa? Contact us! We are the top Hotspring spa parts retailer on the web. We offer:

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!