Pentair MasterTemp 400 Shutting Off

thadius65

LifeTime Supporter
Jul 30, 2011
403
Bloomsburg, PA
I have had my MasterTemp 400 for about 7 years. Has worked flawlessly until yesterday. Normally the LED readout is pool temp, but now it is jumping all over the place from 70's to 100, changing every few seconds. Last evening it started shutting down, then pausing for a bit, a few clicks, maybe start, then stop, digits all over the place. This heater is driven by my Hayward Prologic system, with the Pentair on its highest setting and the Prologic controlling when it gets power based on its temp reading. I have reset for 10-15mins several times. Doing so has it showing pool temp and it starts and runs for about 5mins or so, then starts with the numbers changing and shutting off.

I tend to think this is a logic board, but I have no experience with troubleshooting Pentairs. Any guidance on what else to try and what could be root cause?

Thanks!
 

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JamesW

TFP Expert
Mar 2, 2011
23,144
Check the resistance of the thermistor. It should follow a 10k thermistor chart. If not, it's bad and needs to be replaced.

 
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thadius65

LifeTime Supporter
Jul 30, 2011
403
Bloomsburg, PA
Check the resistance of the thermistor. It should follow a 10k thermistor chart. If not, it's bad and needs to be replaced.


JamesW - Thank you for the tip. I received new thermistor this afternoon. Installed and ran without any deviation from actual temp readings and no issues. $34 and 10minutes of work and we are back in business. Appreciate the awesome guidance!
 
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