I have an older Teledyne Laars ESC 400 heater. I have built in hot tub/pool combo. Heater stopped working 5 years ago so was not used. Finally checked myself using test ladder in manual and discovered gas valve was bad. I replaced gas valve and lit immediately. The problem is it wont go to temp set on control. 100-102 as before. It wouldn't go past 96 deg and after replacing thermister, now it won't go past 88 deg. 135 and 150 hi limits have 12 volts going in and out on both so does that mean that they are okay? Heater exhaust is 275 deg. Control has pool/spa switch but same issue on both. While heating if I turn down temp heater shuts off and if I increase it relights so that seems ok. I'm at a loss on what to check next. Hard to believe both potentiometers would be bad. I have cleaned both with contact cleaner. TIA.