Pentair MasterTemp 400 Heating Light Flashing [5 Easy Fixes]

Pentair MasterTemp 400 heating light flashing is caused when the heater is calling for heat but not igniting.

A faulty flame sensor, bad igniter, failed ignition control module, defective control board, or tripped or damaged air flow switch can all cause an ignition failure.

Read our Pentair MasterTemp 400 troubleshooting guide to stop the heating light from flashing.

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Pentair MasterTemp 400 Heating Light Flashing [5 Easy Fixes]

What is the heating light for? The heating light on the control panel indicates whether the burner is on or not.

So, whenever the burner is lit, this light should be lit as well. When the heating light is off, the burner should be off as well.

When you see that the heating light is blinking, it means that your heater is calling for heat but not firing. We will now take an in-depth look at the causes of ignition failure and how to solve them in a simple and easy way.

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

1. Dirty or Faulty Flame Sensor

Inspect the flame sensor for rust. Check the flame sensor porcelain for any signs of cracking. These may be indicative of a damaged flame sensor. 


To test your flame sensor, bypass it to see if the pool heater fires without it. In case you get successful ignition, it means that the flame sensor is the problem. 

First, you should try to clean up the flame sensor and see if that fixes the problem. Here’s how you clean it.

  • Turn off the circuit breaker or the power to the heater.
  • Completely close the gas valve.
  • The wire that is linked to the flame sensor should then be disconnected.
  • Then, using a Phillips screwdriver, remove the flame sensor from the burner housing.
  • Remove the flame sensor from its mounting.
  • Then clean the black rod of the flame sensor with something abrasive such as emery cloth.
  • Gently wipe the flame sensor with a clean piece of cloth.
  • Using another clean cloth, wipe the flame sensor socket.
  • Return the flame sensor to its original location.

If the heater still doesn’t fire after cleaning the flame sensor, you need to replace it.

2. Bad Igniter

A bad igniter is another common reason for ignition failure. If the igniter fails to produce a spark your heater won’t fire.


You can spot the igniter near the top of the combustion chamber. To replace it follow the steps given below.

Step 1: Turn off the heater’s pump and all electrical power. At the Molex connector, disconnect the Igniter wires.

Step 2: Unscrew the nuts that hold the Igniter Bracket in place. Remove the HSI from the unit with care.

Step 3: To test, check resistance across HSI. At normal temperature, the resistance should be between 50 and 500 Ohm.

Cracks in the HSI that impair performance will change the level of resistance, but they may be too minor to notice.

Step 4: Reverse Steps 1 through 4 to replace the HSI. When you repair the HSI, you also need to replace the Igniter Gasket.

Twist the igniter hold-down nuts till it is tightened at a torque of 20 inch-lbs. Overtightening the HSI base might cause it to break.

Step 5: Turn on the Combustion Air Blower and use a soapy water solution to check for leaks.

3. Failed Ignition Control Module

The ignition control module or ICM controls the ignition process and timing. When the module fails, the ignition system falls apart. A failed ICM can be why your heating light is blinking.


The only way to solve this issue is to replace the ICM. The following instructions will guide you through Pentair MasterTemp 400 Ignition control module replacement.

Step 1: Remove the heater’s left side panel. There are four screws that need unscrewing. 

Step 2: Separate the Max-E-Therm heater from the system. The wires and connectors that connect the legacy ICM should be separated. 

Step 3: Unscrew the older ICM’s two mounting screws. Remove the ICM unit from the heater that came with it.

Step 4: Place the ICM in the heater’s Junction Box chamber. You will find specific instructions on connecting the wires in the manual. Follow the manual to connect the wires while replacing the ICM

Step 5: Then, take off the control panel assembly and reassemble the whole thing. Make sure you can change the control panel without bending over the exhaust vent.

4. Defective Control Board

Almost all of your pool heater’s functions are controlled by the control board. Many issues with your Pentair MasterTemp 400 can be caused by a malfunctioning control board, including an ignition failure. 


If you have a defective control board, you need to replace it. Request a replacement control board from Pentair MasterTemp 400 customer service.

For detailed instructions, watch this video on how to replace the control board on a Pentair MasterTemp 400.

5. Tripped Or Damaged Air Flow Switch

The airflow switch is a protective feature that monitors the vacuum pressure inside the blower housing to ensure that the combustion air blower is working. The airflow switch is connected to the ignition module and is factory configured. 

Only if the air pressure switch and the other safety switches are closed, the ignition module will not turn on. So, a tripped or malfunctioning AGS may cause ignition failure.


You can reset the AFS by ensuring that the blower is working properly and nothing is blocking the airflow. In case the blower is fine, you may have a defective airflow switch. Replace it. 


What does the service system mean on the Pentair pool heater?

Service system light being on means that there is an error with your heating system. The heater may self-diagnose the issue for you by turning on the error lights or displaying error codes.

Why does my pool heater keep shutting off?

Generally, a clogged filter is the most common reason why pool heaters keep shutting off. Insufficient voltage supply, low gas pressure, shorted AGS, faulty temp sensor or defective control board may also be liable.

How do I clean my pool heat exchanger?

Remove the heat exchanger from its mounting screws and place it on a clean surface. Using a soft-tipped brush, apply a degreaser to the heat exchanger. Allow time for the degreaser to loosen any remaining soot. Using a garden hose, clean the heat exchanger.

Can low pH damage pool heaters?

Yes, it can. Metals in your pumps, filters or other heater parts corrode in low pH water.

What temperature should the pool heater be set at?

Setting the pool heater’s thermostat to between 78°F and 82°F will be comfortable for most people. If you live in a hotter environment, it might be a little cooler, and if you live in a milder one, it might be a little warmer.


Hopefully, after reading our Pentair pool heater troubleshooting guide, you have a clear idea of what to do.

You can leave us a comment below if you have any queries regarding your Pentair MasterTemp 400 heating light flashing. We will answer your question ASAP.

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 .

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