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Nothing happens but topside says SN, Sn1 or Sn2?

Spas that simple go to light flashing or SN means the pcboard sees failure of one of the sensors. On some spas it will tell you which. It will say SN1 if the High Limit sensor has failed or SN3 if the temp sensor has failed.

On many models it will simply say SN. This can be a challenge to determine which sensor it is. The field tech will take a quality ohm meter and check the resistance on each sensor to what it should be. The incorrect one with be the failure. If you can't do that, the most common sensor to fail is the OH sensor. The temperature sensor is a distant second.