Question about Pentair Mastertemp Heater

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Heater will not come on.

Water temp reading on wall mounted control reads over 100 degrees, but the temp on the Mastertemp panel reads 45 which is correct. I can't get the calibration feature to go below 100 on the controls. I checked the thermistor with a new one and they both read the same. The heater will run when I set the set point to over 100, but shuts off when the two or three degrees raise the control panel reading.

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  • Pentair Master
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Controller temp sensor bad

Posted on Oct 07, 2017

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Posted on Jan 02, 2017

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UglyPoolGuy
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SOURCE: have pentair easy touch controls just installed

If the Easy Touch is reading the correct temperature, but the heater is not - you need to double check the temperature of the pool water with a standard pool thermometer.
Other things to consider:
1) The water is traveling too slowly through the heater. If the water is heating too quickly, then it will shut it off. If it takes a while for this to occur, then this would not be the issue.
2) Thermostat/thermistor is incorrectly reading the temperature of the water prematurely shutting the system down.
Make sure you really are checking the temperature of the water and matching this temperature up with one of the two readings that you are getting. Don't just take the word of the displays on either unit.

Posted on Dec 10, 2010

  • 4 Answers

SOURCE: have pentair easy touch controls just installed

Go to "calibrate" on your new unit and lower the spa "set at " temp a little

Posted on Dec 15, 2012

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1 Answer

Master Temp 400 reading 32 degrees on LED read out and the remote control is reading 62 degrees. What can cause the difference?


The control panel has its own temp sensor which is a probe into your plumbing. It's typically found between the pump and the filter and is held on by band clamps.

The heater has its own temp sensor found on the header manifold area.

i doubt the water is 32 degrees so I assume the heater temp probe is bad, it needs to be replaced, it's plugged into the circuit board and threaded into the header of the heater.

The two sensors should match or be within one degree of each other for ideal operation.

The reason they have separate temp sensors is to protect the system.

Nov 26, 2016 | Pool & Spa

1 Answer

My Pentair Mastertemp 400 temperature is reading 20 degrees too low. I've replaced the thermistor already. Any ideas?


bummer hi limit wont do that but tep sensor will or control board also make sure water is flowing freely,
you will want to check the manifold bypass and thermal regulator

May 29, 2014 | Pentair Mastertemp Heater

1 Answer

My Pentair Mastertemp 400 temperature is reading 15 degrees too low. I've replaced the thermistor already. Any ideas?


Your problem sounds like a faulty control board. I replaced the high limit switch, the thermistor, and the control board in an attempt to resolve an erratic temperature reading (plus and minus 1 degree).
It caused the heater to turn off and on constantly when the target temp was reached. My successful solution involved replacement of the two brown wires that connect the control board to the thermistor. The wires were ok but the connector(s) were not. The thermistor output is very small and the contact must be solid in order for the control board and thermistor to communicate. As for the price of the board...it is closer to $300. The one I purchased (and didn't need) was about $275. Seems like you have done everything except change out the board. It doesn't sound like your problem involves corroded wires. Good luck

May 29, 2014 | Pentair Mastertemp Heater

1 Answer

I have a Pentair Mastertemp 400. It worked fine for 3 years until the temp sensor went out and both (internal and external) were replaced about a year ago. It then worked fine for about another 7-8...


You might have a much bigger issue than the temp sensor. When the heater is running hold the "pool" or the "spa" button for 12 seconds. This will show the exhaust temp, if the temp is rising above 375 degrees with in 1-2 mins then your not getting the heat out of the heat exchanger.
One other thing to check would be an external by-pass. If you have an external by-pass make sure the by-pass is set correctly. Opened or closed to much can cause the problem your having.

Mar 04, 2017 | Pentair MasterTemp 400000 BTU Pool and Spa...

3 Answers

Have pentair easy touch controls just installed new water temp sensor. spa is 88 degrees but master temp heater shuts off because it reads 104. what should i check? the thermistor/


If the Easy Touch is reading the correct temperature, but the heater is not - you need to double check the temperature of the pool water with a standard pool thermometer.
Other things to consider:
1) The water is traveling too slowly through the heater. If the water is heating too quickly, then it will shut it off. If it takes a while for this to occur, then this would not be the issue.
2) Thermostat/thermistor is incorrectly reading the temperature of the water prematurely shutting the system down.
Make sure you really are checking the temperature of the water and matching this temperature up with one of the two readings that you are getting. Don't just take the word of the displays on either unit.

Dec 10, 2010 | Pentair Mastertemp Heater

1 Answer

Thermostat reads 120 degrees but pool is only about 75


Your temp sensor has gone bad. It is a small tube that slides into the water manifold with two wires comming from it. Follow these to the control board and then unplug them. Now just get a new one and replace. This should fix the problem. Good luck.

May 15, 2010 | Pentair Mastertemp Pool & Spa Heater Dual...

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