Why Pentair Mastertemp 400 Won’t Turn On [Solved]

Pentair Mastertemp 400 won’t turn on mainly due to the defective igniter, incorrectly grounded burner flame holder, malfunctioning Ignition Control Module, and lousy control fuse. 

Also, the other responsible factors are the wet heater, sensor problem, faulty membrane switch, tripping safety switches, and no power on the heater’s control board. 

pentair mastertemp 400 won't turn on

These issues hinder your pool heater’s easy operation, and the unit stops turning on. Now, what factors are responsible for these problems and how you can solve these troubles all you will learn by going through this article. So, keep reading till the end. 

Pentair Mastertemp 400 Won’t Turn On [10 Troubleshooting Tips]

You are pressing your Pentair Mastertemp 400 heater Pool and Spa button. But your heater’s LCD screen displays nothing and the heater is not turning on mainly for the following reasons:

Note: You can also read how to fix Pentair heater error codes problems.

1. Faulty Igniter

The damaged or wet igniter can’t ensure high resistance. Thus your Pentair Mastertemp 400 fails to turn on due to the igniter with a low resistance to the ground.

bad  igniter of pentair masteramp heater

However, the igniter becomes faulty for the following reasons: 

  • Broken, bent, or dirty igniter 
  • Clogged filter 
  • Worn ignition switch contacts 
  • High voltage 
  • Excessive gas valve pressure 
  • Defective control board 
  • Faulty igniter wire 


The defective igniter prevents your Pool heater’s turning on and flame creation and causes a gas leak in your heater by increasing the gas flow.

Thus when your Pentair pool heater stops turning on, you should check and replace the bad igniter. 

The other solutions to fix this issue are: 

  • Replace the damaged or broken igniter, worn-out ignition switch contacts, igniter’s faulty wiring, and the malfunctioning control board 
  • Properly clean the clogged filter 
  • Ensure your heater is not getting high voltage 
  • Reduce the gas valve pressure 

Igniter Replacement Instructions

To replace your Pentair pool heater’s faulty igniter, do the following: 

  • Turn off your pool heater’s gas and electricity supply. 
  • Open your heater’s faceplate, and then find out the pronged igniter. 
  • Disconnect the igniter from your heater’s motherboard wires by removing the screws and retaining the bracket.
  • Get a new igniter, the same as the previous igniter’s part number. And then connect the new igniter to your heater’s motherboard wires. 
  • Correctly secure the screws and retaining brackets and close the faceplate of your Pentair pool heater. 
  • Now the igniter replacement is complete. Turn on your heater and set the desired temperature. 

2. Improperly Grounded Burner Flame Holder

An insecurely grounded burner flame holder is also responsible for preventing your Pentair Pool heater from turning on.

Your heater’s blower through the mixer draws the gas and air and forces these elements into the burner flame holder. 

The sealed heat exchanger surrounds the burner flame holder. And the exchanger out of the flue discharges the exhaust gas.

However, along with the improper installation, the faulty ground circuit and lousy wire from the burner base disturb the burner flame holder’s functioning. 


Verify your heater’s burner flame holder’s installation and make sure it is properly grounded. Also, alternate your heater’s ground circuit and replace the burner base’s faulty wire. 

3. Defective Ignition Control Module

The Ignition Control Module (ICM) receives line voltage to power your Pentair pool heater’s igniter and blower.

Also, the gas valve gets low-voltage power from this ICM. However, the faulty ICM fails to ignite your heater. 

And thus your Pentair Mastertemp 400 stops turning on. Also, your heater displays the error code ERR IGN. The reasons for which the ICM goes bad are: 

  • Power is not going on the ignition module 
  • Ignition lockout 
  • Inappropriate gas flow or pressure 
  • Black residue or dirty build-up 
  • Faulty igniter 


The Ignition Control Module is in charge of your heater’s ignition sequence and timing. So when it goes bad, your pool heater will not start.

To check your heater’s ICM, remove and open the ICM up and inspect why it goes bad. Then replace the faulty ICM. 

However, to fix the above troubles, follow the ways below: 

  • Replace the bad igniter 
  • Make sure your heater’s ignition control module is getting sufficient power. 
  • Get your heater out of ignition lockout mode. 
  • Ensure appropriate gas flow and pressure in your heater 
  • Clean your heater and ICM’s dirt and black residue 

Ignition Control Module Replacement Instructions

Replacing the faulty ICM is a must so that your Pentair Mastertemp 400 pool heater again turns on perfectly. The easy replacement steps are: 

  • Take off your Pentair pool heater’s left side panel. 
  • From the Max-E-Therm heater, disconnect the system. Also, disconnect the legacy ICM’s wires and connectors. 
  • Now disconnect the previous ICM unit on your pool heater. 
  • On your pool heater’s Junction Box chamber, insert the new ICM.
  • Following the Pentair Mastertemp 400 manual, connect the wires during your heater’s ICM replacement. 
  • Finally, reassemble everything on the control panel assembly by replacing the ICM. 

4. Bad Control Fuse

The lousy control fuse inside your heater’s silver box and the power interface board are other reasons which hinder your heater from coming on.  

bad  control board fuse of pentair masteramp heater

The blown 1 ¼ amp fuse in your heater’s control panel creates a short on your heater’s system board. When you are trying to turn on your heater, the fuse can blow immediately. 


Even if your Pentair Masteramp heater’s terminal board gets power, the heater won’t turn on, which means the faulty fuse is creating an issue.

First, test the terminal board’s fuse to ensure your heater’s continuity. Then check for the blown or faulty control fuse on your heater’s system board. And then replace the fuse or the entire system board. 

5. Wet Heater

Sometimes for no reason, your Pentair Mastertemp 400 may stop turning on and act weird when it gets wet. Your wet heater may sometimes work or sometimes refuse to turn on.

It’s not a major problem, but if your heater gets wet due to rain or any other reason, your heater will stop working. 

When you try to turn on your wet heater, its igniter doesn’t fire. Also, the sparking igniter’s working condition becomes vulnerable. Thus your heater won’t turn on. 


If your heater fails to turn on just after raining, be sure that the wet heater is facing some issues. Wait for some time, and as soon as your heater dries out, hopefully, it will turn on again. 

6. Sensor Issue

Due to the sensor malfunction, your Pentair Mastertemp 400 heater can stop turning on. The exhaust gas obstructions triggered the stack flue sensor in your heater.

Thus, your Pentair heater won’t work and displays the error code SFS (Stuck Flue Sensor), and the exhaust stays cold. 

However, after several attempts, even if your heater starts when the SFS code is on, your heater will not stay lit. And also the exhaust temperature will remain below 250 degrees. The causes of sensor malfunction are: 

  • Dirty or corroded sensor 
  • Faulty thermal regulator or thermal regulator seats incorrectly 
  • Sensor’s faulty connections 
  • The unusual sound coming from the sensor 
  • Defective or dirty heat exchanger, coil 
  • Incorrect fuel supply pressure and volume 


Disconnect your Pentair Heater’s sensor and terminal. Then checks the continuity across the sensor. The sensor continuity or resistance should be between 3.5-4.0 megaohms. 

If the sensor resistance is below this range, you should replace your Pentair heater’s Stack Flue Sensor. The other effective solutions are: 

  • Replace the corroded or faulty sensor, defective thermal regulator, the sensor’s bad connections, malfunctioning heat exchanger, coil
  • Properly clean the sensor, heat exchanger, and coil. If needed, replace the heat exchanger.
  • Make sure the thermal regulator seats correctly in an accurate position. 
  • Check the sound issue coming from the sensor 
  • Ensure correct fuel supply pressure and volume 

7. Defective Membrane Switch

Your Pentair Mastertemp 400 fails to turn on due to the faulty membrane switch. Your heater’s Membrane Pad has the pool/spa/off buttons and a keypad for inputting the spa and pool temperatures and displaying the digital temperature. 

defective membrane switch 

Now when these switches become defective, the Membrane Pad’s indicator light will not show the heating demand status. And thus your heater will stop starting. The Membrane switch becomes faulty for the following reasons: 

  • The membrane switch gets stuck. 
  • Faulty Membrane Pad 
  • Membrane switch’s damaged or faulty wiring. 


When your Pentair Mastertemp 400 refuses to turn on, check your heater’s Membrane Pad and see whether the pad is displaying the digital temp or not. If not, be sure the faulty Membrane switch is the culprit. 

Thus replace the malfunctioning or stuck Membrane Pad, Switch, and switch’s damaged or bad wiring. 

8. Tripping Safety Switches

Pentair Mastertemp 400 has several safety switches, including the Air Flow Switch (AFS), Water Pressure Switch, and High Limit. Each switch performs different safety functions such as: 

Tripping Safety Switches 
  • Air Flow Switch: Monitor the blower housing’s negative vacuum pressure and ensure the operation of the combustion air blower. 
  • Water Pressure Switch: Prevent the burner from firing when water flow is restricted. 
  • High Limit Switch: This switch shuts off the heater and opens the electrical circuit based on the water temperature. 

All these switches are your heater’s safety devices. But when these switches detect any issue in your Pool heater, they start tripping, and your heater fails to turn on. 


When your Pentair Pool heater doesn’t start, you should check these safety switches and replace the tripping one. 

9. No Power On Heater’s Control Panel

Your Pentair Mastertemp Pool and Spa heater can refuse to turn on if its control panel gets no or insufficient power to run.

no power on pentair masteramp heater’s control panel

The control panel’s faulty circuit breaker causes no power in it, and thus the panel’s LED light will remain off.  

Your heater stops working or blowing when a lower volt or no volt comes to the control panel out of the transformer. However, the other causes of the control panel’s malfunction are:

  • Faulty sub-panel 
  • Defective control board 
  • Tripping circuit breaker 
  • Overloaded thermal breaker 
  • Damaged, & defective transformer
  • Low voltage supply in the control board 


If your heater fails to turn on or ignite, first check whether your heater’s control panel is getting power or not. Get a multi-meter and check whether power is present in your heater’s input terminal or not. 

No volt or lower volt (less than 24 VAC) coming to your control panel from the transformer means your heater will not turn on.  To fix your heater’s control panel’s hitch, do the following: 

  • Replace the control board’s faulty sub panel, overloaded thermal breaker, tripping circuit breaker, malfunctioning control board, and faulty transformer
  • Ensure your heater’s transformer supplies proper power or voltage to your heater’s control board.

Note: Checking the voltage is quite risky and has accident potential. So be careful while tasting the transformer’s voltage. 

10. Blocked Filter

The pump basket blockage or clogged filter causes water flow resistance. Thus, your Pentair Mastertemp 400 fails to operate and turn on.

Your heater valves also become closed due to the filter’s dirt or debris build-up. Moreover, water can’t move at a normal rate through the clogged filter. 

However, your heater’s filter becomes blocked for the following reasons: 

  • High water pressure in Filter 
  • Plastics, paper products, oils, or mulch accumulated in the Filter 
  • Not cleaning the Filter frequently. 


Over time, the slow accumulation of debris creates a clog in your heater’s Filter. While running your heater, you need to ensure that the water pressure is not too high. Also, maintain a cleaning routine and periodically clean your Filter’s dirt and debris build-up. 

Pentair Mastertemp Pool Heater Filter Cleaning Instructions

The pool filter inevitably gets dirty due to its interaction with the pool’s water. So regularly clean your pool filter to avoid a clog. However, the easy filter-cleaning steps are: 

  • Gain access to your pool heater’s filter by removing the filter cover 
  • Unscrew and lift out the filter 
  • Now spray out or tap out your heater’s filter.
  • Remove and adequately clean the filter’s remaining large debris. 
  • Now reassemble the filter, put its cover, and turn on your heater. 


Why does my Pentair heater keep turning off?

Your Pentair pool heater keeps shutting off due to inadequate water flow, a closed valve, a dirty filter, a control board malfunction, low gas pressure, a faulty igniter, and a faulty ignition control module. 

Where is the thermal regulator on a Pentair Pool heater?

A thermostatically activated valve is the thermal regulator. The thermal regulator is located between your heater’s inlet and outlet fittings, behind the Inlet/Outlet Manifold’s threaded plug. 

What does E05 mean on the Pentair Pool heater?

Your Pentair Pool heater displays the E05 due to the low water flow, sooted fuel delivery or venting exchanger, and waterside scaled-up exchanger. 

How do I check the exhaust temperature on my Pentair Mastertemp?

Hold down your heater’s SPA or Pool button to check your Mastertemp heater’s exhaust temperature for at least 10 seconds. However, your heater will show the exhaust temp. And 290-350 is the normal temperature range. 

Bottom Line

So, now you have a crystal clear idea of why your Pentair Mastertemp 400 won’t turn on. Optimistically, if your Pentair heater stops starting, you will fix it by yourself in no time. 

Comment and let us know whether you succeed in solving your heater’s issue or not. We are always here to help you. 

About William

William is the founder of Fireplacehubs.com. 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.

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