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My Mastertemp 400 will not heat past 89 degrees. what are some of the problems causing this?

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Your thermal regulator most likely needs to be checked and replaced. Also check your inside temperature

Posted on Dec 20, 2016

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Posted on Jan 02, 2017

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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

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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

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I have the same exact problem with this unit. Obviously getting gas to the unit, because it runs and pumps out some serious heat for a short time, but then kicks off. Did anyone ever figure this out or did you buy a new heater? :-)

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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.

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Long story but bingo just at the end. The thermistor is generaly the problem with what you describe. it is an inexpensive part and easy to replace. You will not generaly find this in your book. Your E5 code is just because the unit hasn't started yet. Good luck.

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Service light on


I just had the same problem resolved, my mastertemp 400 would run up the temp to 82F and then cutoff, the service light flashes and then 50 seconds later the heater would restart and run for 2 minnutesbefore shutting down again and relighting in 50 seconds. Repeating until the temp setting is reached.

I had the pool guy out 3 times until he found a bad thermal regulator valve (kit #77707-0010). Once replaced the heater runs great.

He told me that it's my pool chemestry that is to blame, I had it checked (like I do weekely by me and monthly by the pool supply store) and it perfect??? I'm now questioning what caused the corroson and stop it from coming back?

I hope tis helps.

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Oven will not heat past 220 degrees


Check your bake element, I had the exact same problem and found the bake element had burned in half. The broil element comes on during preheat and kicks off at around 220 degrees.

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