I have a five year old SR2000NA in-ground natural gas heater that has performed brilliantly for the past four years. Unfortunately, this year when I opened the pool it will not operate. The display shows code "E01" and the SERVICE HEATER incicator is illuminated. I have searched everywhere and cannot find any reference to this specific code, so am hoping someone might have an idea of what it indicates. Any help you might be able to offer would be greatly appreciated.