Display shows SN3; spa may shut down heating and pumps intermittently

Safety: Turn OFF power at the GFCI breaker before removing panels or touching wiring.

• Check water temperature: Use an accurate thermometer in the spa water. If actual water temperature is normal (around setpoint) but the display is erratic, a sensor issue is likely.
• Inspect sensor seating: Open the equipment bay and locate both M7 sensors on the heater tube. Ensure each sensor is fully threaded in and snug so it makes good contact with the heater tube. Do not overtighten.
• Compare sensor readings: With power OFF, disconnect both sensors and measure resistance. They should be within a few hundred ohms of each other at the same water temperature. A large difference indicates one sensor is failing.
• Swap sensor positions (diagnostic): If accessible, swap the two sensors in the heater tube and reconnect them to the same board inputs. If the error changes from SN3 to SN1/SN2 or follows a specific sensor, that sensor is defective.
• Replace the faulty sensor: Install a new Bullfrog-compatible M7 sensor in place of the defective one, using a new O-ring. Refill and bleed air if you drained water.
• Reset and monitor: Restore power, clear the error by power cycling if needed, and monitor operation for at least one full heat cycle.

If SN3 persists with new sensors and correct installation, the control pack may have an internal calibration or logic fault; contact a Bullfrog Spas service center.