Pentair Mastertemp 400 Wrong Temperature [5 Easy Solutions]

The fact of Pentair Mastertemp 400 wrong temperature, the faulty temperature sensor is the leading cause behind this issue.

Other possible reasons are defective thermal regulator, faulty ignition control board, or failure of the high limit switch.  

pentair mastertemp 400 wrong temperature

To fix the temperature displaying malfunctions of the heater, you have to concentrate on the temperature sensor first.

If that doesn’t work, you will have to fix the other responsible factors. Go through our comprehensive and effective guide to get details solutions for this issue.

Pentair Mastertemp 400 Wrong Temperature [5 Easy Solutions]

Throughout this article, you will find the quick working solution guide on “pool heater reading the wrong temperature.” All the well-proven solution steps will assist you in fixing the temperature issue in a very short time. So, let’s start.

1. Faulty Temperature Sensor

If the pool heater fails to read the water temperatures of the pool, be sure a faulty temperature sensor would be the first reason.

The temperature sensor is referred to as a thermistor on various pool heater models, which is a small tube in the figure.

The small tube slides into the manifold adapter inlet. The main function of the thermistor is to control the heater’s operation. 

Also, it monitors or senses the return water temperature. After sensing the temperature, it sends the resistance reading to the control board.

Then, the readings translate into a digital reading. After long use, the sensor may be corroded and burnt out. In some cases, the sensor is covered by any hard obstacles that prevent reading the temp.

Thermistor failure displays impossible temperature fluctuations on the board or sometimes incorrect temperature readings. Now you have to check out all these factors:


  • Locate the temperature sensor of your pool heater. 
  • Now, examine the sensor. There should not be any corrosion or damage.
  • Also, make sure the sensor is not covered by anything and is free of obstacles.
  • If needed, replace the bad thermistor.  
  • Make sure the sensor is properly mounted. 

2. Defective Thermal Regulator

If the sensor is quite in good condition and hooked to the control board easily, there is the suspect of a defective thermal regulator of the pool heater.

Now, if the display is still showing the wrong temperature, you have to check out the thermal regulator this time. The thermal regulator locates between the inlet and the outlet of the heater.

 It regulates the water flow operation through the heat exchanger. For this, the water outlet temperature can easily be maintained.

Failure of the thermal regulator activates the high-limit switches that turn off the heater.


  • Turn off your heater and unplug it from the electric connection.
  • Now, unscrew the heater and locate the thermal regulator.
  • Pull out the regulator and check it deeply. 
  • If it is corroded or damaged, replace it without any delay.
  • Now, check if the heater operates as it should.

3. Reset The Pool Heater

If still, the pool heater is reading the wrong temperature, there may have glitches in the heater functions. Because of power hitches on the heating element, it will display the incorrect temperature.

Most of the time, the display reports extremely high water temperature, which is known as a high-temp lockout.

This time, you have to reset your pool beater function to diminish this lockout. Here are the needed steps to reset your pool heater.

How To Reset Pentair Mastertemp 400 Pool Heater

  • In order to reset the pool heater, move to the circuit breaker set up for your heater.
  • Flip the breaker “Off” position.
  • Your heater will turn off, including all the ongoing functions.
  • Leave the heater in this position for some while to let it rest.
  • Then switch the breaker into the “ON” position.
  • Your heater will come on.

4. Malfunctioning Of Control Board

You have reset the pool heater, and still, if there have been malfunctions in the temperature reading, the next suspect is the bad control board itself.

The control panel board assembly is located on top of the heater. It controls the major functions of the heater based on the heater input.

Control board failure can be caused by a short-out circuit due to material separation from the essential heating element. 

Other major reasons for the defective control board are improper handling and aging, which become less accurate and display the incorrect temperature. Anyway, you have to replace the bad control board to resolve an erratic temperature reading.


  • Check out the control board first. 
  • You will find two brown wires which connect the control board to the thermistor.
  • Verify your connector first and ensure they are in good condition.
  • In the case of corroded wires, replace the wires.
  • But if the control board is defective, replace it.

5. High Limit Switch Failure

High-limits switch is an essential element for any pool heater. If your pool heater is displaying incorrect temperature, there would have a higher possibility of a high limit switch failure. 

Basically, if the temperature exceeds during the ignition, the temperature limit switches activate and shut down the heater to prevent overheating.

But in the case of a faulty high limit switch, the display shows the wrong temperature and fails the heater to turn on. Here, you have to bypass the high-limit switch. 


  • To bypass the high-limit switch, you will need a jumper wire.
  • Make sure the heater is unplugged.
  • Now, connect the wires with both terminals of the switch.
  • There should have a good connection between the jumper wires and the switch wires.
  • It will assist in bypassing the switch.
  • Turn the heater on. If it comes on, be sure the high-limit switch is damaged.
  • Replace the faulty high-limit switch. 


Why is my pool heater not reaching the set temperature?

If your pool heater is not reaching the desired temperature, be sure there have restrictions on water flow through the unit. Also, because of the improper setting of the controls, the heater triggers the same issue.

Where will I find the temperature sensor on my pool heater?

To locate the temperature sensor, move to the filter pump of your pool heater. You will find the sensor in between the filter pump and the filter. 

Do pool heaters have a thermostat?

In almost all digital pool heaters, there has a thermostat. This element is connected to the knob on the control panel. The main function of the thermostat is to regulate the water temperature of the heater.

Wrapping It Up

Expectantly, now you have a crystal clear idea about the temperature displaying issue of the pool heater. Hopefully, you will easily fix your heater issue by following our above guide.

Still, if you have queries on any facts of the guide, don’t hesitate to let us know through comments. We will be glad to assist you.

About William

William is the founder of He has real life practical skills in fixing smoker & heating appliance issues. He loves to share his knowledge & helps others in fixing their appliances & saving their money. William firmly believes that anyone can repair his or her unit with the correct guidance & knowledge. See more at about us.

2 thoughts on “Pentair Mastertemp 400 Wrong Temperature [5 Easy Solutions]”

  1. my pentair heater wont ignite. replaced the igniter and icm. i have 230 volts coming in but only 75 volts going to the igniter. can you help with this problem. i am confused.


Leave a Comment