Pentair Mastertemp 400 Short Cycle


New member
Apr 5, 2021
Hello, I have a Mastertemp 400 that is short cycling (heats for 30-40 seconds then stops for 30-40 seconds, rinse and repeat).

I have replaced the Control Board, HLS, AGS and Stack Flue Sensor. The Thermistor appears to be functioning (reporting correct water temp at the heater display) and the thermal regulator opens and closes when placed in and our of hot water. The water pressure switch is operating normally. I opened the manifold and found a piece of debris lodged in the manifold bypass valve that was holding it in the open position so I removed that. After further inspection of the MBV it appears to be intact and functioning. There are no errors reported on the display and no LED's are illuminated on the back of the control board.

Any ideas of what else if could be, I feel like I have replaced all the electrical components that would cause this except for the Ignition Control Module and Airflow switch.


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

What diagnostic LED lights on the back of the board when the heater shuts down?

While running, press and hold the On button will display the exhaust temperature display.

Here is what the normal exhaust temperature should be:
  • Below 250 degrees...very low
  • 250 to 290 ..................low
  • 290 to 350.................acceptable
  • 350 to 480 ..................high
  • Above 480...................error/shutdown.
Note: HD models can be up to 75 degrees higher. HD models use a cupro nickel exchanger and the efficiency is slightly lower resulting in less heat transfer and more waste heat.

If the exhaust gas temperature is higher then normal then water may be bypassing the heat exchanger through a broken bypass valve.

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