Pentair MasterTemp 400 Service Heater Light On [5 Easy Fixes]

When you find your Pentair MasterTemp 500 service light on, you need to remove the screws from the top panel of your heater and flip to see which one of the error service lights is illuminated. There are five of these lights, starting from PS, HLS, AFS, AGS, and SFS LED lights.

Each of these LED lights being lit is an error that your heater is self-diagnosing. You need to fix these error codes by servicing your heater. Afterward, the service heater light should no longer be on.

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Pentair MasterTemp 400 Service Heater Light On [5 Easy Fixes]

In this section, we will take a look at the meaning of each of these error codes, their causes, and simple solutions. So keep on reading our Pentair MasterTemp 400 common problem to fix your heater.

1. PS – Pressure Switch Error

This error light will be lit at the back of your control panel when the pressure switch of your Pentair is open. This code leads to an overflow of water.


You must ensure that there is no unwanted water flow in the case of the MasterTemp 400 pressure switch error. You may easily do this by turning the water pressure switch.

In case the water flow throughout the heater continues, you must fix the water flow line to stop it. You may need to call in a professional to get your heater checked. 

2. HLS – High Limit Switch Error

The high limit switch is a safety device that turns off the heater when the temperature gets too high. This error LED lights up when the high limit switch is stuck open.

When the temperature drops below 57°C, this problem develops. Extreme weather, incorrect water flow, and other factors can all contribute to this.


You need to ensure that the water passing through the heat exchanger is above 57°C. The temperature sensor will receive the reading it requires in this manner.

The error light should no longer be on. In case it is on, you may need to replace the high limit switch

3. AFS – Air Flow Switch Error

When the airflow switch is open, the Pentair MasterTemp 400 AFS LED will light up on the back of your control board.

In less than a minute after turning on a Pentair heater, the system’s airflow is optimized. If there isn’t enough airflow at this time, the heater will shut down on its own.


Make sure the fan isn’t impeding airflow to fix this problem number. To do so, you’ll need to:

  • Turn off the heater.
  • The blower fan’s cover should be removed.
  • Look for any clogs or obstacles in the region.
  • Ensure that the fan is entirely depleted.
  • Make sure the blower is securely fastened in its place.
  • Position everything back in its appropriate place.

The issue with your Pentair service heater light on should be solved at this point.

4. AGS- Automatic Gas Shutoff Error

Due to a low flow rate or a defective pressure switch, the gas flow in your Pentair heater may cut off on its own. When this happens, the service heater light may be lit.


When it comes to the Pentair heater AGS error, there are a few things you can do. These are-

  • Check to see if the water flow rate is higher than the minimum.
  • Increase or decrease the water flow rate.
  • Make sure the water temperature is less than 60°C.
  • The high-limit switch should be replaced.

If this does not resolve the problem, you will need to get your heater serviced by a professional.

5. SFS- Stack Flow Switch Error

When the temperature of the stack flue exceeds 248°C, the ERR SFS fault code appears. A broken thermal regulator or an improper fuel supply are the most common causes of this problem.


When this error occurs, you must address a few issues. Follow the instructions below:

  • Turn the Pentair heater on.
  • There should be a thermal regulator there.
  • Now, ensure that the thermal regulator is where it should be.
  • Handle any blockages, soot, or leaking in the heat exchanger coils.
  • Ascertain if the gas pressure is adequate.

After completing these steps, your Pentair service heater light should no longer be on.


Why does my Pentair pool heater cycle on and off?

Cycling is frequently caused by large limit mistakes. A high limit indicates that the heater’s water is becoming excessively hot. This is frequently caused by a malfunctioning temperature regulator. The high limit issue might also be caused by a faulty internal bypass.

How do I turn off the gas to my pool?

Remove the heater’s door and turn the gas valve counterclockwise to OFF. Then, press the knob at PILOT while continuing to turn. Then turn off the manual shutdown valve situated outside the heater to cease the gas flow to the heater.

How do I light the PILOT on my pool heater?

Place the knob in the “PILOT” position and press and keep it there. The pilot diaphragm is manually opened, and gas is sent to the pilot assembly. Press in the “click-lite” 3-4 times while keeping the knob down. This should get the pilot going.

Why is my pool heater not coming on?

A clogged swimming pool filter is frequently the cause behind a Pentair heater not turning on. A blocked filter or pump basket can obstruct water flow to the point that the heater can’t function.

How do you adjust the pressure switch on a pool?

Lower the minimum required pressure by slowly turning the screw to the left, counter-clockwise almost to the point where the heater turns on. The Adjustment Wheel on other adjustable pressure switches is turned with your thumb and finger.

Hopefully, you now know what to do when you spot the Pentair MasterTemp 400 service heater light on.

In case you have questions about your Pentair pool heater, you can comment below. We will help you out to the best of our abilities. 

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 .

2 thoughts on “Pentair MasterTemp 400 Service Heater Light On [5 Easy Fixes]”

  1. Liked your video and hope you are able to advise or lead me in the right direction.
    Have tried finding repair company to no avail in my part of UK.
    Problem: The electricity does not come on at all. Have checked fuse, plug and wires seem to be all in tact. As I am a simpleton(!) are you able to offer any advice?
    Pentair Mastertemp 125 pool heater.
    +44 7871754326 mobile

    • Hi Nigel

      Check the control fuse & the fuse inside the silver box.

      If they are okay, inspect the pump flow to the heater & ensure there are no obstructions & the water runs to the heater at least 35 GPM (Gallons per minute).

      If you get the water flow is low, then check the heat exchanger & clean it.

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

      Best of luck!


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