Control panel displays SN1 and spa will not heat; temperature reading is extremely low or shows dashes

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

• Power cycle the spa: Turn the GFCI breaker OFF for 2–3 minutes, then ON. If SN1 clears and does not return, it may have been a transient error.
• Inspect the sensor cable: Remove the equipment compartment door. Locate the heater tube on the Bullfrog control pack (Balboa-style pack). Find the M7 sensor leads (usually two identical sensors threaded into the heater). Trace the cable from the sensor labeled as Sensor 1 to the control board. Look for cuts, rodent damage, or pinched insulation.
• Check sensor connection: Gently unplug the Sensor 1 connector from the control board, inspect for corrosion or bent pins, then firmly reconnect. Do not pull on the wires; grasp the plug body.
• Compare sensor resistance: With power still OFF, disconnect both M7 sensors and measure resistance with a multimeter. At typical spa water temperatures (80–100°F / 27–38°C), both sensors should read similar resistance (usually 10k–30k ohms depending on temperature). If Sensor 1 is open (infinite resistance) or drastically different from Sensor 2, it is defective.
• Replace the sensor if needed: If Sensor 1 is faulty, drain water below the heater tube level, unscrew the sensor from the heater tube, and install a new Bullfrog-compatible M7 temperature sensor with the supplied O-ring. Hand-tighten only; do not overtighten. Reconnect the cable to the control board.
• Restore power and test: Refill if needed, bleed air from the pump unions if you opened the plumbing, then turn the breaker ON and verify that SN1 is cleared and the spa heats normally.

If SN1 persists after replacing the sensor and verifying wiring, the sensor input on the main control board may be damaged; contact a Bullfrog Spas dealer or qualified spa technician.