Pentair Mastertemp 400 Common Problems [11 Easy Solutions]

Pentair Mastertemp 400 common problems include displaying the wrong temperature, low water circulation, leaking water, water scaling, and corroded heat exchanger.

Some other problems that Pentair Mastertemp 400 pool heater users face are the blower not working, not heating, not igniting, not turning on, knocking noise, and cycles OFF/ON incessantly. 

pentair mastertemp 400 common problems

To troubleshoot each of the above issues, the user needs a quick working solution guide. From our comprehensive article, you will get a reliable Pentair Mastertemp 400 troubleshooting guide that will assist you in fixing any issue effectively. Let’s start.

Pentair Mastertemp 400 Common Problems [11 Easy Solutions]

Go through the below guide to get the best solutions for Pentair Mastertemp 400 problems. Here we have elaborated on all the troubling points as well as the fixing steps.

1. Pentair Mastertemp 400 Heater Not Turning On

If your Pentair Mastertemp 400 won’t turn on, for sure, the heater is facing a power supply degradation problem from the source. Other major reasons include:

  • Insufficient water flow
  • Wrong valve settings
  • Broken safety switches 
  • Damaged heater transformer
  • Incorrect temperature settings in the thermostat

Here are the needed steps to fix Pentair Mastertemp 400 no power:


  • Before all else, ensure a fluent power supply to the heater.
  • Along with that, check the water flow to the heater. There should have at least 35 GPM (gallons per minute).
  • For that, clean out the water filter first.
  • Replace the defective heat exchanger if needed. 
  • On the thermostat, set the correct temperature. The temperature should always be higher than the pool heater. 
  • Measure the voltage of the heater transformer. Replace it if you find no voltage there. 
  • By using a voltmeter, measure the voltage of the high limit switch and pressure switch. Replace the bad switches
  • Hopefully, the heater will turn on.

2. Pentair Mastertemp 400 Displaying Wrong Temperature

Pentair Mastertemp 400 wrong temperature seems to be the most common issue on this pool heater. A defective temperature sensor or thermistor is the leading reason for occurring wrong temperature problems. 

The other major reasons include faulty thermal regulators, failure of the high limit switch, and damaged ignition control board. Anyway, fix the problem by following the below methods:


  • First, locate the thermistor of your pool heater.
  • Remove the hard obstacle near the sensor. Replace the defective thermistor.
  • Check out the thermal regulator as well. You need to replace the corroded or damaged regulator.
  • If still, the problem is persisting, reset the heater by turning the breaker to “ON” and “Off” positions.
  • Verify the control board is in well-working condition. If not, replace it.
  • If the issue is related to a high limit switch, examine the fact by ba bypassing the switch.

3. Pentair Mastertemp 400 Low Water Circulation

If the water level is too low than normal on your pool heater, it would be a possibility of a clogged water filter.

Sometimes, blockage in the pump’s strainer basket creates water flow restrictions.

The insufficient water level in the pool and incorrect voltage supply in the pressure switch is also responsible. 


  • Clean out the water filter first.
  • Don’t ignore the strainer basket near the pump. It should be damaged and crack-free for fluent water flowing.
  • Now, measure the voltage reading on the water pressure switch. Replace the defective pressure switch.

4. Pentair Mastertemp 400 Leaking Water

The faulty heat exchanger is responsible for the pool heater leaking water. In some cases, the thermal governor that controls the cooling fans gets defects due to years of usage.

Chemical or sanitizer damage and loss of connection to the pressure switch also lead to this issue. 


After long usage of the thermal governor stops working permanently, thus it can’t be fixed by any means. There is nothing, but you have to replace it to fix the leaking issue.

5. Pentair Mastertemp 400 Scaling Water

Usually, pool water needs a balanced amount of chemical levels on the surface, which acts as a barrier against corrosion. But when the amount exceeds the limit, the heater catches it as a problem.

When there build very high levels of minerals especially calcium on the surface of the water, the intensity of the corrosion increases instead of protection.

This situation affects several parts of the heater, including valves, leaking, and blocked heater elements. 


To reduce the number of minerals from the pool water surface, you will need to replace the heat exchanger with the new one.

6. Pentair Mastertemp 400 Heat Exchanger Corrosion

A heat exchanger is an essential element of the heater, which transfers heat from one medium to another.

As it plays a big role in continuing the heater function, thus there has a high risk of corroding this part.

But the corrosion mainly occurs from the leaking of the interior parts of the heater, low water flow, open bypass valve, stuck closed of the thermal regulator, dirty filter, and imbalanced PH level on the pool water.

After persisting on this issue, you will hear a sound like a bumping sound from the heater.  


  • Clean out the filter first to resolve the low water flow issue.
  • Check the pH of the water and make sure it is balanced.
  • Use a heat exchanger made of cupro-nickel to resist corrosion.
  • Replace the defective thermal regulator.
  • Also, check out the bypass valve and replace if it is stuck open. 

7. Pentair Mastertemp 400 Not Heating

Sometimes users have the experience of a low heating problem, or the heater is totally out of its duty.

Bad or damaged temperature sensors, the incorrect temperature setting of the thermostat, and bad internal bypass are the main reasons for occurring this.

Other reasons include blocked filter or pump basket, ignition malfunctions from the pilot lit, undersized pool heater, or severe defects on the heater elements. 


  • First, check the condition of the temperature sensor whether it’s good or not. Replace the bad thermistor. 
  • Clean the water filter and pump basket to eliminate the water flow
  • Ensure the correct temperature of the thermostat. Set the temp higher than your pool water.
  • Check out the internal and external bypass systems. Replace the bad ones.
  • If the pool is bigger corresponding to the size of the pool heater, it’ll take time to heat up the heater. 
  • Check out the other heating elements. Repair or replace the pilot lit. 

8. Pentair Mastertemp 400 Not Igniting

Ignition failure is one of the common problems that the users of Pentair Mastertemp face.

Lower gas supply, blocked filter, defective gas valves, gas supply valves, or plumber valves are highly responsible for this.

A faulty transformers, bad igniter, and incorrect thermostat temperature setting are regarded as some of the other major reasons behind this issue.

Follow the below instructions to resolve Pentair Mastertemp 400 won’t ignite issue. 


  • First, power cycle the heater to erase all the unwanted factors.
  • Ensure enough gas supply on the heater. For that, clean out the gas orifice and burner system. Check out the fuel level as well.
  • Clean the clogged water filter to ensure a fluent water flow.
  • Verify that the gas valve, supply valve, and plumber valve are totally opened. 
  • Check out the power of the transformer whether it’s all good or not. Replace the defective one. 
  • Replace the bad igniter and defective flame sensor from your heater.
  • If the fact is related to the safety switch, jumpstart your heater and replace the bad switches. 
  • Finally, set the thermostat temperature correctly.

9. Pentair Mastertemp 400 Blower Not Working

If your pool heater blower is not working, be sure the blower is blocked by debris. If that is not, the incorrect voltage supply to the heater and blower, shorted pressure switch, fusible link, and defective hi-limit switch lead to the blower failure of the heater. 


  • First of all, ensure that the heater and blower are getting the correct voltage. 
  • Inspect the blower and clean out the stuck debris of the element.
  • Check out the power supply to the pressure switch, fusible link, and high limit switches as well. Replace if there have any defects.
  • Ensure that the transformer has at least 24 volts.
  • Now, check out the voltage supply to the control board. Replace the bad board.

10. Pentair Mastertemp 400 Cycles On And Off

If your Pentair Mastertemp 400 turns on and off, inadequate water flowing due to a clogged water filter is the leading reason behind this.

Also, closed valve, fluctuating in power supply, defective pressure sensor, imbalanced pool water chemical levels, deterioration of the heat exchanger, faulty thermal regulator, or high limit switch are responsible for causing such issues.

Follow the below instructions to resolve Pentair Mastertemp 400 keeps shutting off and on the issue:


  • Make sure all the valves are opened and in good working conditions.
  • Clean the water filter properly to prevent water flow restrictions.
  • Adjust the chemical level of the pool water.
  • Fix all the power issues.
  • Then do a power cycle on the heater.
  • Check out the pressure sensor, thermal regulator, or high limit switch. Replace all the defective ones.
  • Now check out the heat exchanger as well. Replace if you find any chemical or sanitizer damage there. 

11. Pentair Mastertemp 400 Knocking Noise

Knocking sounds occurs due to the clogging in the skimmer or the strainer basket of the pump or the pool.

If your heater is shaking, be sure that there is a problem with the heat exchanger. It could be plugged, or the bypass valves could be sticking.

The possibility of water chemistry issues like calcium buildup in the heat exchanger, low water flow, and faulty pressure switch can create strange sounds in your heater. To solve this sound issue, check through the below solutions.


  • The thermal regulator can be stuck closed. Check through it.
  • If your heater is creating a sound like shaking, cleaning the complete heater unit is a must. Clean the skimmer or the strainer basket of the pump.
  • Make sure enough water flow by cleaning the filter.
  • Replace the faulty pressure switch
  • If still there are strange sounds or shaking, contact the manufacturer of the heater because it could be risky for you now.


What is the function of the High Limit Switch on Pentair?

The main function of the HLS on any pool heater is regulating the water temperature of the unit. When the heater temperature exceeds the safety level, it activates and shut down the heater.

How does the thermistor work on the pool heater?

A thermistor is mainly regarded as a temperature sensor. It has a big role in controlling the heater operation. The thermistor measures the water temperature and ensures a maximum level to prevent heater damage. 

Why is my Pentair pool heater not coming on?

Glitches in the power supply are the root reason behind this. Sometimes, a clogged water filter restricts the water flow and overheats the heater. Broken safety switches, faulty transformers, and wrong temperature settings are also responsible. 

Wrapping Words:

Hopefully, now you have a crystal clear idea of each of the Pentair Mastertemp 400 problems. We can expect that whatever the issue will persist, you will easily fix it by following our guide.

But now, if you have any questions, leave a comment below. We are always ready for your assistance.

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.

4 thoughts on “Pentair Mastertemp 400 Common Problems [11 Easy Solutions]”

  1. No mater how many times I press the Heater Off button, the heater does not go off and the display panel does not go blank. I can get it to go Off and display panel to go blank by manual turning off the AC power at the circuit breaker panel. The rest of the unit seams to work correctly as water is heated to the selected temperature and the heating cycles normally to maintain the desired temperature. The heater is a 400k model 460736 installed about 2006. I do not see a procedure for not being able to use the OFF button.

    • Hi William Martin

      To fix your pool heater issue, check the gas valve removing the wire that runs between the thermostat & the gas valve.

      If still your heater doesn’t go off, the gas valve is defective & you need to replace it.

      And if the heayer shuts off, the gas valve is ok. Then check the control board & fix it.

      Hopefully, your problem will be solved.

      Wish you all the best.

  2. Sir, I have a natural gas Pentair 400 Master Temp new about 5 years ago. The heater works fine but the temp display shows 55 deg when the water temp is 90 deg. The thermister, sensors, and Board are working. Could the problem be the regulator? If so where is it located and how much does it cost. Thanks for your help. Tech did everything but change the Regulator.

    • Hi Bryan

      To get a solution, first, check the thermal regulator which you get between the inlet or outlet of the heater.

      You may need to replace it if you get it corroded.

      If it doesn’t work, contact the Pentair customer support center.

      Best of luck!


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