Mastertemp 400 Temperature Issue


New member
Nov 23, 2021
I have a Pentair Mastertemp 400 that is about 3 years old. Recently the spa has not been heating like usual. The spa will only heat to 91 degrees. I checked the temperature setting on the heater and it set to 104. After reading on the forums I believed the culprit could be the thermistor. I replaced it and have the same issue with the new thermistor. The heater seems to believe the temperature is 104 but it’s still only heating to 91. I am not seeing any error codes or lights on the back of the control board. I was considering replacing the thermal regulator but it doesn’t seem like that would effect the temperature reading on the unit. Could this be a faulty control board?


Gold Supporter
TFP Guide
Jul 21, 2013
Northern NJ
Pool Size
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
Welcome to TFP.

You need a multimeter and a 10K resistor.

The temperature sensor is a 10K thermistor. You can measure its resistance with a multimeter.


For a test replace the thermistor with a 10K resistor.

If the temperature stays at 76F then you still have a problem with the new thermistor.

If the temperature drops you probably need to check the wiring from the thermistor to the board, and maybe replace the board.



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