Pool and Spa
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Yes the Sn code is the sensor set that is either miss wired or damaged. The temp sensor controls the thermostat settings 80-104 degrees and the high limit sensor is there to back up the temp sensor if the water temp were to get up to 110 degrees then the high limit would trip and kill power to the heater so you do not scald yourself in the water or melt the plumbing lines.
In most cases I tell my customers to replace both sensors when ever they have to replace the heater in their spa as it usually causes issues once they are removed and replaced again. I would remove and replace both of them just to be sure.