Pentair MasterTemp 400 Wrong Temperature [5 Easy Fixes]

Your Pentair MasterTemp 400 wrong temperature being displayed can be caused by a faulty thermistor, wet heater parts, broken thermal regulator, bad manifold bypass, and malfunctioning control board. 

Read our article to troubleshoot your Pentair MasterTemp 400. 

Table Of Contents

Pentair MasterTemp 400 Wrong Temperature [5 Easy Fixes]

We will now take an in-depth look at the causes of the wrong temperature reading on your Pentair MasterTemp 400 as well as explore the simplest solutions for each one.

Note: You can also read how to fix Pentair masterTemp 400 common problems.

1. Faulty Thermistor

A thermistor is a resistor whose resistance changes with changes in temperature. A faulty thermistor is a common reason for your pool heater reading the wrong temperature.

It is cheaper to fix than some of the other heater part failures that can cause this issue. 


Follow the instructions below for Pentair MasterTemp 400 thermistor replacement.

Take off the back panel to access the thermistor. There are 3 screws to remove before lifting off the panel.

Your Pentair MasterTemp 400 temp sensor location is near the high-limit switch. Remove the wiring connections that go into the thermistor. 

With a ⅞” deep socket, loosen up the thermistor and take it out. Put the new one in and turn it to stabilize.

Tighten the thermistor with a ⅞” deep socket. Reconnect the wires and slide the back panel in its place. Tighten up the screws. 

2. Wet Heater Parts

A lot of times, water gets inside your pool heater. If your heater’s manifold bypass, water flow sensor, thermistor, or control board gets wet, you may get the wrong temperature readings.


Remove the panels from your heater and check each part to see if they are wet. With a blow dryer go over the heater parts that seem wet.

Be careful with the blow dryer and turn it on as low as it will go. Blow from a distance when drying sensitive heater parts like the control board. 

Once the heater is dry, reset it manually. This means, turning off the heater and waiting for 10-15 minutes before restarting it. Afterward, your heater should be displaying the right temperature reading.

3. Broken Thermal Regulator

A broken thermal regulator or thermostat is another reason why you may be getting the wrong temperature on your Pentair MasterTemp 400’s display. 


You can take out the thermal regulator and test its continuity. The Thermal Regulator is situated between the inlet and outlet fittings in the Inlet/Outlet Manifold, behind the threaded plug. 

Use a regular screwdriver and wedge it into the threads of the plug and twist the handle to unscrew the plug. Keep doing that, till the plug comes off along with the thermal regulator.

Take water that is 130° F in a container. Immerse the thermal regulator in the water to see if it opens. In case it opens, you have a working thermal regulator.

Double-test it by immersing the thermal regulator back in cold water. The device should close again if in fine working condition. Replace the thermal regulator if it failed the test.

4. Bad Manifold Bypass

Bad manifold bypass is another reason for wrong temperature readings. A bypass guides the flow of water around the heater, preventing it from passing through it. So, a malfunctioning manifold bypass may result in water entering your heater.


You need to replace your manifold bypass and also the gasket. The process of manifold replacement is pretty simple. 

Remove the front and rear panels, the manifold side cover, and the bolts that hold the manifold in place. The manufacturer’s bolts may be covered in red high-temperature Permatex. Scrap it off. 

To disassemble the manifold assembly, you’ll need a screwdriver and a hammer. Put the new manifold in place.

Alternate tightening of the bolts to compress all of the o-rings and gaskets back into place. Torquing head nuts on a motor is similar.

5. Malfunctioning Control Board

A malfunctioning control can also be the culprit. This is the most expensive one to fix. But without a functioning control board, your heater has no hope of functioning properly. 


Your control board needs to be replaced. Please contact Pentair customer care to get a replacement control board. You may replace the control board yourself if you have some technical knowledge.

  • Remove the two side covers first. Then unscrew your Pentair’s top cover. Because the control board is linked to the top cover, lift it gently. To get to it, you must first flip the cover over.
  • Take a photo of all of the wires leading to the control board. To be able to reproduce each connection, try to figure out where it is formed.
  • The screws that hold the old control panel in place should be removed. Install the replacement control board by screwing it in place. Remove the wires from the old control board one by one and connect them to the new one.
  • Reinstall the top and side covers once you’ve verified that the new control board is working properly.

 The following video should help you out with the process. 


How do you test a Pentair stack flue sensor?

Take a multimeter and set it on resistance testing mode. Place each multimeter prong on the terminals of the stack flue sensor. If the resistance is zero, the stack flue sensor is malfunctioning. The sensor is operating if the resistance is 3.5 ohm or above.

Do pool heaters have thermostats?

Heat pumps feature a built-in thermostat, and most models have twin digitally controlled thermostats. If the water temperature dips by more than two degrees after the desired temperature is reached, the thermostat will turn off the device or turn it back on.

How does water flow through a pool heater?

The heater’s intake has a bypass, which causes water to flow around the heater rather than through it, before passing via the heat exchanger and out. The water then passes via a backflow prevention device before entering the high-concentration chemical chlorinator.

How many kW Do I need to heat my pool?

One liter of water takes 1.16 Wh to heat by one degree, equating to 11.6 kWh for a 10m3 pool. To put it another way, one-kilowatt hour raises the temperature of 862 liters of water by one degree Celsius.

What chemical is used to lower pH in a pool?

Use a pH reducer, a chemical addition manufactured specifically for pools, to lower the pH.  Muriatic acid or sodium bisulfate are the main active components in pH reducers.


Hopefully, you now know what to do when your Pentair MasterTemp 400 wrong temperature reading is displayed on the LED screen.

In case you need any troubleshooting advice regarding your Pentair MasterTemp 400, leave us a comment. We will help you out to the best of our knowledge.

About David

David is a man who loves doing research and finding out the best solution to any type of heating appliance issues like heaters and fireplaces. Whenever he finds the perfect solutions to any issues of heating appliances after doing in-depth research, he loves to share them with his readers. In fact, his aim is to help his readers to fix the issues with their heating appliances easily.More about us .

Leave a Comment