Control panel displays Sn2 and spa may not heat or may shut down

Safety: Shut off the spa at the GFCI breaker before any work in the equipment bay.

• Locate the high-limit sensor: On most Michael Phelps Spas, the Sn2 sensor is the secondary or high-limit sensor threaded into the heater manifold or clamped to the heater tube, with leads going to the control pack labeled SENSOR 2 or HI-LIMIT.
• Inspect the sensor body: Check that the sensor is fully threaded into the heater manifold or firmly clamped to the heater tube and not loose or hanging.
• Check wiring and connector: Follow the cable back to the control pack. Unplug the connector, inspect for corrosion, bent pins, or moisture, then reconnect firmly.
• Look for physical damage: Ensure the cable is not pinched under the equipment bay door or against the frame.
• Test resistance (optional): With power OFF, measure resistance across the sensor leads and compare to the manufacturer’s temperature/resistance chart. Replace if open, shorted, or far off spec.
• Replace sensor if needed: Unscrew or unclamp the high-limit sensor and install a new OEM Michael Phelps/ Master Spas-compatible high-limit sensor. Use thread sealant if specified by the manufacturer.
• Restore power: Turn the breaker ON and verify that Sn2 clears and the spa heats normally.

If Sn2 reappears quickly, there may be an underlying overheating issue (low flow, airlock, or stuck relay) that needs professional diagnosis.