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 the spa: Turn the GFCI breaker OFF for 2–3 minutes, then back ON. If Sn1 returns after startup, continue below.
• Inspect sensor wiring: Remove the equipment compartment door. Locate the heater tube and the two M7 sensors threaded into the stainless heater housing. Follow the cable from the sensor labeled as Sensor 1 to the control pack (Balboa/Marquis control box). Ensure the plug is fully seated and not corroded or wet.
• Check for physical damage: Look for cuts, kinks, or rodent damage on the sensor cable. If damaged, the sensor must be replaced.
• Test sensor resistance (advanced): With power OFF, disconnect the Sensor 1 plug from the control board. Using a multimeter set to ohms, measure resistance across the two sensor pins. At about 77°F (25°C) water, you should read roughly 10k ohms (NTC thermistor). A reading that is open (OL) or near 0 ohms indicates a bad sensor.
• Replace the sensor if faulty: Drain water below the heater tube level or close isolation valves if installed. Unscrew the M7 sensor from the heater tube, install the new sensor with a new o-ring, hand-tighten plus a small additional turn. Reconnect to the control board.
• Restore power and test: Turn the GFCI breaker ON, allow the spa to boot, and verify that Sn1 is cleared and heating resumes.

If Sn1 persists after replacing the sensor, the issue may be with the Marquis/ Balboa control board input circuit and a qualified spa technician should evaluate the control pack.