Control panel displays Sn1 and spa will not heat or shows incorrect water temperature

Safety first: Turn off all power at the spa’s GFCI breaker and verify with a non-contact voltage tester before opening the equipment bay.

• Power cycle the spa: Turn the GFCI breaker OFF for 2–3 minutes, then back ON. If Sn1 returns after startup, continue below.
• Access the equipment bay: Remove the front service panel to expose the Michael Phelps Spas control pack (Balboa/Master Spas style pack).
• Inspect the primary temperature sensor: Locate the sensor leads labeled SENSOR 1 or TEMP on the control board. Follow the cable to the sensor well threaded into the heater manifold.
• Check connections: Unplug the sensor connector from the control board, inspect for corrosion or moisture, clean gently, and firmly reconnect.
• Check for damage: Look for cuts, pinches, or melted insulation along the sensor wire where it passes through the cabinet or near the heater.
• Test the sensor (if you have a meter): With power still OFF, disconnect the sensor and measure resistance across the two sensor leads. Compare to the temperature/resistance chart in the Michael Phelps Spas or Balboa tech manual (typically ~30kΩ at 77°F/25°C). If reading is open, shorted, or far off spec, the sensor is bad.
• Replace if faulty: If damaged or out of spec, unscrew the sensor from the heater manifold (have a towel ready for minor water seepage) and install a new OEM Michael Phelps/ Master Spas-compatible temperature sensor. Tighten snugly but do not overtighten.
• Refill/bleed if needed: If you lost water, top off the spa and bleed air from the heater union by slightly loosening the union until water seeps out, then retighten.
• Restore power and test: Turn the GFCI breaker ON, allow the spa to boot, and verify that Sn1 is cleared and temperature reads correctly.

If Sn1 persists after sensor replacement, the control board’s sensor input circuit may be defective; contact a qualified spa technician.