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?