I recently bought a house with a in ground pool/hot tub combo that includes a Hayward H400 pool heater. It was a bank owned house where the pool had been sitting without filtering for probably 6 months. After getting everything cleaned up, having the system gone through, I fired it up and the heater cycled right on and i left it on for about 3 days heating the hot tub only. After leaving it off for about a week I came back and now it won't cycle on and gives me the "LO" diagnostic code on the LED read out. Pressure is good going into the unit so all I can figure is its 1 of a couple sensors that could either be bad or giving me a bad reading. I live in Arizona where the water is pretty dang hard. Where should I start in diagnosing this before calling the pool guy? Which if any of the sensors most commonly fail on these? Any help would be great!