Pentair Mastertemp 400 Common Problems

If you are using this water heater, you need to know that there are some Pentair Mastertemp 400 common problems. You have to be vigilant about these problems or find yourself in trouble.

Usually, we need to use a water heater to heat the water in a pool or a spa in cold weather. For this reason, we need reliable water heaters. Pentair Mastertemp 400 is one of the most reputable water heaters.

Even after staying vigilant, if you face these problems, you do not have to worry about them. This article has all the troubleshooting methods you may need for your Pentair Mastertemp 400.

Pentair MasterTemp 400 Error Code

There are a few of these problems that you might face. First, you need to understand the error codes. Here is a chart below explaining them.

Error CodeReason
E05Problem with the stack flue sensor
E04When there is a problem with the fan sensor
E03The thermal fuse sensor is showing issues
E02Hight limit sensor has a problem
E01Temperature sensor showing problems
AFSThe switch of airflow is showing problems
PSPressure switch error
SFSStack flue sensor showing issues
HLSHigh limit switch has a problem
AGSThis happens when the automatic gas shutoff is having issues.
Common Error Code For Pentair Mastertemp 400

Pentair Mastertemp 400 Common Problems

Pentair Mastertemp 400 Common Problems

The most common problems are listed down below:

  • Corrosion and Boiling of Heat Exchanger
  • Water Circulation is Low
  • Sweating
  • Scaling Waters
  • Control Board’s Voltage Supply Low
  • Icy Residue 

Corrosion and Boiling of Heat Exchanger

The worst-case scenario for a water heater is corrosion of the heat exchanger. When this happens, your Pentair Mastertemp 400 water heater will not be working correctly. You do not want that. So, this needs t to be fixed.

If you catch yourself having this issue, there will be symptoms. The main symptom is that you will hear bumping sounds from your heater. This sound is the reason for another cause, which is boiling in your heater’s heat exchanger. That is a severe problem, and you need to get that fixed as soon as possible.

You will find that a few leading causes make this issue possible. 

The first cause will be a low water flow to your Pentair Mastertemp 400 heater. 

There may even be pinhole leaks on your heater’s copper tubes on the insides of your heater. This will cause corrosion as airflow and water will turn into rust.

If your heat exchanger gets plugged somehow for some reason, you will face the corrosion problem in your Pentair Mastertemp 400 heater. 

Some pools will have saltwater in them. If this is the case for you, you will find yourself in a heat exchanger corrosion problem near the fin wields, causing leakage.

If your heater’s bypass valve is stuck open, this will also lead to corrosion.

Pools or spas are used for years and years. Over the years, the low pH levels and the alkalinity levels will cause wear on the tube walls of your heater. This will eventually result in corrosion.

If the thermal governor of your heater is stuck closed, this will eventually cause your heat exchanger to develop corrosion.


Prevention is better than any kind of cure always. The causes you read above need to be prevented before they occur. Or else, you will witness corrosion of the heat exchanger of your heater.

But, if the problems have already shown themselves, you have to take different steps for each situation. 

Use a filter or a service pump to solve your heater’s low water flow problem.

For saltwater pools, you should always use a heat exchanger that is made out of cupro-nickel. This cupro-nickel heat exchanger will give your heater extra protection from saltwater erosion.

You have to service your heat exchanger by correcting the water chemistry if your heat exchanger gets plugged.

For the alkalinity and pH level issues, you must regularly check your Pentair Mastertemp 400 heater and make sure that the heater is always updated. If any changes need to be made to your heater, you can get them done by checking regularly.

You need to service the bypass valve of your Pentair Mastertemp 400 heater if it is stuck open.

Replace the Thermal Governor if it is stuck closed.

Water Circulation is Low

There are several reasons why the heater can be low water circulation. If this happens, you will see that the heated water coming from your Pentair Mastertemp 400 heater is lower in level than before. 

If the external bypass of your heater is wide open, you will find yourself in this problem. Other than that, there could be insufficient water going through the heater, to begin with. 

You might not even turn on your machine. This is a common problem. Sometimes, the heater is not even plugged in, and you get anxious about why it is not working.

Sometimes, the switch that controls water pressure might get damaged. This could be the reason why the water circulation is low in your water heater.


All you have to do is prevent the issues mentioned above. 

Check to see if your water heater is turned on or not. If not, then plug it into a reliable power source and turn it on. Make sure that there are no lint traps, worms, or dirt near.

Clean the filter if it is not clear already. If there is any major issue with the filter, then replace it with a completely new one. Do not keep the external bypass open at any cost.

Now check the pressure switch and replace it if it is damaged. Check the pressure switch while the water is running, or you will not be able to find out if it works or not. Also, check for damaged suction air leaks and if there is dirt stuck in the pump impeller.


When the thermal governor fails, sweating occurs. This is when the heat exchanger leaks.


Sweating occurs due to thermal governors not working correctly. After years of usage, this issue may arise. 

You cannot easily fix a thermal governor. That is why you need to buy a new one to replace the old one with.

Scaling Waters

Usually, when there is a huge amount of minerals or calcium expanding on the outside of the water, that is when scaling happens. 

Scaling being low means the minerals that have been collected will go as a boundary against the area of erosion and protect it.

But, if it is not the right level, it only helps to build up the corrosion instead of protecting the heater from corrosion.

You will find at least three impacts of scaling. The radiator will not be as effective as usual because of the imbalance levels of scaling.

Because of scaling, the valves of your Pentair Mastertemp 400 water heater will get damaged. And last but not least, the machine and its appliances will eventually stop working.


There is no cure for this problem. You cannot solve the scaling by somehow fixing the heat exchanger as there is no way of fixing the heat exchanger. That is why you need to replace the heat exchanger with a new one.

Control Board’s Voltage Supply Low

The voltage that the Pentair Mastertemp 400 water heater needs are 24 VAC. Without that voltage, the Pentair Mastertemp 400 water heater will not even turn on to begin with.

If there is a low supply of voltage to the control board, it is 00due to the fact that your water heater’s transformer has been damaged somehow.


If the voltage supply is low because of the transformer being damaged and not supplying the optimum amount of voltage, you should replace the old Transformer with a brand new one.

Icy Residue

Sometimes, there is an icy residue left on the outside of the air coil. If airflow gets blocked somehow, this residue will be left behind.


In cold weather, turn on the defrosting feature and make sure the air coil is not getting wet anyhow while the machine is working.

Final Words

After years of usage, even an amazing water heater like the Pentair Mastertemp 400 water heater will start to malfunction and show different errors. It is nothing to be afraid of. You just need to fix a few things or replace a few parts.

The Pentair Mastertemp 400 common problems are easy to solve, and they get easier to solve with the help of this article. Just stick to what this article says, and you will be good to go.

Abdulla Zatar
About Abdulla Zatar

Hi, I’m Abdulla! Your go-to tech troubleshooter! Sharing my knowledge so that you can fix your tech issues with ease. Let’s make fixing things easy and fun!

1 thought on “Pentair Mastertemp 400 Common Problems”

  1. What causes transformer failure. Will nearby lightning strike cause?
    Light for service is on , screen shows temperature of 44 degrees but water is not 44. Changes 43-46.
    Flow switch seems fine. Plenty of water running through


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