Display shows F2 and heater does not operate; circulation pump may still run.

Safety first: Turn OFF power at the breaker before touching any wiring or sensors.

• Reset the system: Turn power OFF for 5 minutes, then ON. If F2 returns, proceed.
• Locate the temperature sensor:
  • Open the equipment bay access panel.
  • Find the USSPA heater tube; the main temperature sensor is typically threaded or inserted into a sensor well on the heater housing.
• Inspect sensor and wiring:
  • Check that the sensor is fully inserted and snug in the heater tube or sensor well.
  • Follow the sensor cable back to the control board; look for cuts, abrasions, or rodent damage.
  • Unplug and replug the sensor connector on the control board to clear oxidation.
• Basic resistance test (for experienced DIYers):
  • With power OFF and sensor unplugged, use a multimeter set to kΩ.
  • Measure resistance across the two sensor leads. At around 25°C (77°F), most spa thermistors read ~10 kΩ (check USSPA documentation for exact spec).
  • If reading is open (OL), near 0 Ω, or far outside expected range, the sensor is defective.
• Replace the sensor if faulty:
  • Drain water below the heater level if the sensor is in direct contact with water.
  • Unscrew or pull out the old sensor, install the new USSPA-compatible temperature sensor with a new O-ring if applicable.
  • Route the cable away from hot surfaces and moving parts, then reconnect to the control board.
• Restore power and test:
  • Refill spa if drained, bleed air from pumps, then power ON.
  • Verify that F2 is cleared and that the spa heats normally.