nordic impulse spa. water was below skimmer line, added water pump and jets work, no air in system but heater will not engage now. worked fine for 2 years without any trouble. display works shows 59 degrees water temp, lights work, pump control two stage works just no heat. any thoughts. could be the temp sensor, electronic board or the heater has just failed 1 kw 120v without a wiring diagram and onh value of the sensor can't proceed without calling the service tech.