Problem with Sta Rite Max-E-Therm High Effiency - Gas Heater - 400,000 BTU Model SR400NA. When the heater turns on, the display shows a pool temperature of 109-110, which means I cannot get the heater to blow. The water temp is actually 75 degrees so the unit is malfunctioning. The unit is only four years old. What needs to be replaced?

SOURCE: Sta Rite Sr333na Max E Therm Pool & Spa Heater gauge stuck?

Anytime the heater is reading the water temp to be above what it actually is, especially 126, the problem is the Thermistor, or Temp Sensor, for the heater. This is a very easy part to replace. The Thermistor is located on the manifold, on the left side, between the top two bolts.

Sorry this is a late response! Hope that it helps!

SOURCE: Water Temprature read 126 on heater and will not Fire

This is a problem with your thermister (heat sensor) You will find this on the left side of the water manifold about half way down. I'm not sure but I think it is a small tube that slides into the manifold and if you follow the wires back to the board it just plugs in. The part is reasonable priced for a heater part and very easy to install. Good luck.

SOURCE: Ref: Sta-Rite Max-E 400. Pool temp is about 75

Your temp sensor has gone bad and will need to be replaced. It's a small tube that slides into the water manifold and you can follow the wires back to the board. Just unplug and set a new one in place.