I received a E05 code on my SR200HD pool heater which is the Stack Flue Sensor. Replaced the sensor. Heater ran for about 6 hours and up to temp. The next time the heater started up, it shut down and the Service Heater light came on. The SFS LED on the back of the circuit board was lit. I replaced the stack flue sensor again. Same thing. It ran until up to temp. Next time it fired up, same result. The E05 error code is NOT showing. Only the LED on the circuit board. My system is 14 years old. What could be causing the SFS to go bad? Or is it something else? Does the heat exchanger need to be descaled?