I have a Mastertemp 400 that is not working. IT starts up and runs fine for a minute or two then shuts down and the service heater light comes on and the SFS led is lit . I have had this problem before and replace the thermal regulator and it will work. I have put two new thermal regulators in and it still will not stay running. If I remember right my exhaust temp was around 380 degrees when it was running. The local pool supply store say they think is my heat exchanger. I do have fairly hard water, but before I rip into changing the heat exchanger I was wondering about the stack flue sensor or maybe replacing the high limit switch.