Control panel displays Sn1 and spa will not heat or may shut down

Safety first: Turn off power at the GFCI breaker and verify with a non-contact voltage tester before opening any panels.

• Power cycle: Turn the spa off at the breaker for 2–3 minutes, then restore power to see if the code clears. Temporary glitches can trigger sensor errors.
• Inspect sensor wiring: Remove the equipment compartment door. Locate the primary temperature sensor plugged into the Balboa or Master Spas control pack (often labeled TEMP or SENSOR A). Check for loose, corroded, or wet connectors. Reseat the plug firmly.
• Check sensor body: Find the sensor in the heater tube or sensor well. Ensure it is fully inserted and not cracked or swollen. Look for leaks around the sensor grommet.
• Resistance test (if you have a multimeter): With power OFF and sensor unplugged from the board, measure resistance across the two sensor leads. Compare to a 10k NTC chart (e.g., ~10kΩ at 77°F/25°C). If reading is open, shorted, or far off expected value, the sensor is bad.
• Replace sensor if faulty: Order a compatible Master Spas temperature sensor matched to your control pack (Balboa VS/EL or MS pack). Remove the old sensor from the heater tube, install the new one with the supplied o-ring, and tighten only snugly to avoid cracking.
• If code persists: If a known-good sensor still shows Sn1, the issue may be a failed input on the control board. At this point, contact a qualified spa technician to evaluate the control pack.