Control panel displays SN2 and spa will not heat; temperature reading is extremely high or shows dashes

Safety: Shut off the spa at the GFCI breaker before touching any internal components.

• Reset the system: Turn the breaker OFF for 2–3 minutes, then ON. Observe if SN2 returns immediately or after a few minutes of operation.
• Locate Sensor 2: Open the equipment bay. On Bullfrog Spas using M7 technology, there are two identical sensors threaded into the heater tube. Identify the one wired as Sensor 2 (refer to wiring diagram on the control box cover if needed).
• Inspect wiring: Follow the Sensor 2 cable from the heater tube to the control board. Look for abrasions, melted insulation near the heater, or rodent damage. Ensure the cable is not pinched under the equipment door.
• Clean and reseat connector: Unplug the Sensor 2 connector from the control board. Check for moisture, corrosion, or bent pins. Dry gently if damp and reconnect firmly.
• Test resistance: With power OFF, disconnect both sensors and measure resistance with a multimeter. Both sensors should read nearly the same value at the same water temperature. If Sensor 2 reads open (OL) or is far off from Sensor 1, replace it.
• Replace Sensor 2: Drain water below heater level, unscrew Sensor 2 from the heater tube, install a new Bullfrog-compatible M7 sensor with a new O-ring, and reconnect to the control board.
• Power up and verify: Restore power and confirm that SN2 is cleared and the spa heats normally without error.

Persistent SN2 after sensor replacement usually indicates a control board fault; professional diagnosis is recommended.