I have a LAARS LT 400 natural gas heater for the spa and pool. About a month ago I turned the heater on to use the hot tub and the temperature never got over 80. I had a pool guy come out and he said it was either the flame sensor rod or the thermistor. He changed the flame sensor rod and it did the same thing so he said it was the termistor. I replaced the thermistor over the weekend and it still only gets to 80. Any ideas on what else the issue might be? I have no error lights on the top.