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

Safety first: Shut off power at the GFCI breaker before working in the equipment bay.

• Confirm code: Verify that the topside control actually shows Sn2 and not Sn1 or HL.
• Locate Sensor 2: Open the equipment compartment. On Marquis spas using Balboa M7 technology, there are two identical sensors threaded into the stainless heater tube. Trace the cable from the second sensor (often labeled or color-coded) to the control pack.
• Check connections: Ensure the Sensor 2 plug is fully seated on the control board and that there is no corrosion or moisture on the connector. Dry gently if needed.
• Inspect cable: Look for pinched, abraded, or chewed insulation along the sensor cable. Replace the sensor if the cable is compromised.
• Measure resistance: With power OFF, unplug both sensors and measure resistance across each sensor with a multimeter. Both sensors should read very similar values (around 10k ohms at 77°F). If Sensor 2 is significantly different, open, or shorted, it is defective.
• Replace sensor pair if mismatched: If one sensor has been replaced previously with a non-matching part, replace both M7 sensors as a matched pair to ensure proper operation.
• Reassemble and test: Reinstall sensors with new o-rings, reconnect to the board, restore power, and verify that Sn2 clears and the spa heats normally.

Note: Persistent Sn2 after sensor replacement may indicate a failing Marquis/ Balboa control board and should be diagnosed by a professional.