I have a service light on my Sta-Rite pool heater (model SR400NA). I had a pool guy look at it and he said it was most likely a sensor, and he would have to trouble shoot the heater. I took my ohm meter and measured all the sensors with the leads removed. The Termistor and the stack Flue sensor both showed as open cuircits. Would this be the expected reading if these are not working? and would this cause the heater to keep shutting off?
one other thing you might want to check would be the pressure switch, if you shut the power down to the heater, then disconnect the two wires going to the pressure switch that senses water flow, turn your pump on and check across the two terminals on the pressure switch, you should have continuity there when checking with a meter, or 0 ohms resistance, if you don't have continuity across the pressure switch then the switch is bad or you're not getting enough water flow through the heater to activate the switch