Pentair MasterTemp 400 Turns On And Off [7 Easy Solutions]

Your Pentair MasterTemp 400 turns on and off due to a faulty temperature regulator, damaged ignition control module, bad high limit switch, malfunctioning internal bypass, broken thermocouple, incorrect or loose wiring, and failed control board.

As your Pentair heater turns on and off, you have come to the right place for a solution. Keep reading this article on Pentair MasterTemp 400 troubleshooting and fix your heater ASAP.

Table Of Contents

Pentair MasterTemp 400 Turns On And Off [7 Easy Solutions]

In this section, we will take a closer look at the causes of your Pentair heater cycling on and off as well as provide simple solutions for each issue.

1. Faulty Temperature Regulator

Your heater’s thermal regulator directs the water flow in a way that the heat exchanger outlet temperature stays over 120°F. If this component of your device is faulty, your heater may keep shutting off each time it turns on.


To ensure that your thermal regulator is the cause of the issue, you need to remove it for inspection. This heater component is located in between the inlet and outlet connections of the manifold. You can find the thermal regulator behind the threaded plug.

Insert a Phillips flathead screwdriver inside the plug’s threads. Twist the screwdriver handle to loosen the plug. Continue this motion till you completely remove the thermal regulator as well as the plug.

Fill a jar halfway with water at 130°F. To see if the thermal regulator opens, submerge it in water. You have a working temperature regulator if it opens.

Re-immerse the thermal regulator in cold water to double-check it. If the gadget is in good working order, it should close again. Replace the thermistor if the test reveals a problem.

2. Damaged Ignition Control Module

The ignition control module or ICM coordinates the timing and sequence of the ignition process. If the ignition control module is damaged, a bunch of issues may pop up with your heater, including the heater turning on and off. 


To stop your heater from cycling on and off, you have to replace the ignition control module. Follow the instructions below to replace the ICM.

Step 1: To access the ICM, first, remove the left side panel of your Pentair heater. You will need a screwdriver for this.

Step 2: You then need to take apart the Max-E-Therm heater from the system. Disconnect the wires to the ICM.

Step 3: Two mounting screws hold the ICM in place. Unscrew them to take the ICM off the heater.

Step 4: Now take the new ICM and put it in the Junction Box chamber. This Pentair MasterTemp 400 manual should help you connect the wires correctly to the ICM. 

Step 5: The final thing you need to do is, take apart the control panel assembly and put it back together. You have to ensure that the control panel can be changed without bending the exhaust vent.

3. Wet Or Bad High Limit Switch

Water in the high limit switch causes a short to earth, restricting the supply of current or voltage that can reach the gas valve.

A faulty high-limit switch, on the other hand, can misread the heater temperature and cause your Pentair MasterTemp 400 to start and stop.


By removing the side cover of your machine, you can access the high-limit switch. It is held together by three screws. Jump the high-limit switch and operate your heater without it to see if it’s the source of the problem.

Your heater’s high-limit switch is corrupted if your heater continues to run without shutting down. If it’s wet, try air drying it before restarting the heater. If necessary, replace the high-limit switch.

4. Malfunctioning Internal Bypass

The internal bypass helps maintain the correct exit temperature of the water when it goes through the heat exchanger. A defective internal bypass can be why your Pentair MasterTemp 400 keeps shutting off.


To resolve the problem, you may need to change the internal bypass. This is how you go about doing it.

Step 1: Turn off both the filter pump and the heater’s electricity. Outside, close the Manual Gas Valve. Close the isolation valves if the heater is below the pool’s water level to prevent the pool from draining.

Step 2: Open the Pressure Relief Valve to drain the heater. Remove any covers, doors, or panels to obtain access to the Bypass Access Plug.

Step 3: Using a 7/8″ open-end wrench or an adjustable wrench, turn the Pipe Plug Tool counter-clockwise to open the Bypass Access Plug. You now have access to the Internal Bypass Valve.

Step 4: Take the internal bypass valve off and rebuild the manifold bypass access plug by reversing steps 1-3.

5. Broken Thermocouple

The thermocouple’s job is to check the temperature of the water to see if it has reached the desired temperature. Your heater may cycle on and off if it is receiving inaccurate information.


The sensor can be tested with a multimeter. It should read 10,000 ohms at 77°F  or 25°C. You can search for various test values in the Temperature vs Resistance Conversion Chart for 10k Thermistors. If the thermocouple is broken, it must be replaced.

6. Incorrect or Loose Wiring

This is a common cause of pool heater failure. Insufficient voltage supply can be caused by faulty wiring, causing the heater to turn on and off. A faulty capacitor or a snagged wire may also be the cause.


Your heater’s wiring has to be inspected. Replace any damaged wires after double-checking all of the wiring connections. Make that all of the connections are secure. The following Pentair MasterTemp 400 wiring diagram may assist you.

7. Failed Control Board

Your heater may be shutting down due to a defective control board.


To make certain that the control board is faulty, use a multimeter to test it. Customer care at Pentair can assist you in obtaining a new control board to replace the old one. Hire a local professional for replacement.


How do I test my pool heater thermostat?

Ensure your thermostat is set to a temperature that is greater than the temperature of your pool. The thermostat is in good working order if increasing the temperature causes the heater to switch on. Otherwise, a multimeter can be used to see if the thermostat is receiving power.

Is it cheaper to heat a pool with gas or electricity?

In terms of cost, electric swimming pool heaters are considerably different from gas heaters. Because of their complexity, the devices are more expensive upfront, and the cost of installation is also higher. Electricity, on the other hand, is usually less expensive in the long run.

How do you adjust the temperature on a Pentair pool heater?

To choose between Pool and Spa, press POOL/SPA. The current set temperature is displayed on the display. To increase or decrease the temperature of the water setpoint, press Up/Down.

How much does it cost to run a 400 000 BTU pool heater?

To heat your pool, an average pool heater with 300,000 to 400,000 BTUs will cost $3.30 to $4.40 per hour.

How do I file a Pentair warranty claim?

To begin filing a claim, you’ll need your service team account number. If you don’t have access to this number, contact 800-831-7133 and follow the warranty administration procedures.


Trying the above things when your Pentair MasterTemp 400 turns on and off will hopefully fix your heater.

If you are facing any other issues, you can give our article on Pentair MasterTemp 400 common problems a read. 

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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