Control panel shows F3 and spa may overheat or not heat correctly; heater is disabled.

Safety first: Turn OFF power at the breaker. Allow the spa water and equipment to cool if overheated.

• Check for low flow conditions:
  • Remove and clean the filter cartridges; dirty filters can cause heater tube hot spots.
  • Verify that the circulation pump is running and that there is strong flow from the circulation jets.
• Locate the high-limit sensor:
  • Open the equipment bay and find the heater tube.
  • The high-limit sensor is usually a separate thermistor or manual reset device mounted on or near the heater housing.
• Inspect and reset (if manual reset type):
  • Some USSPA heaters use a manual reset high-limit with a small red button. If present, press the button firmly until it clicks.
  • Check wiring for burns, discoloration, or loose spade connectors.
• Test or replace sensor:
  • If F3 returns immediately after reset and good flow is confirmed, the high-limit sensor may be defective.
  • With power OFF, disconnect the high-limit sensor and test resistance similar to the main thermistor (values should be close at the same temperature).
  • If readings are out of spec, replace with a USSPA-compatible high-limit sensor or reset device.
• Inspect control board if sensors test good:
  • Look for burned traces or components around the sensor input.
  • If the board is damaged or F3 persists with known-good sensors, the main control board likely needs replacement.
• Restore power and monitor:
  • After repairs, power ON and verify normal heating.
  • Monitor temperature for a full heat cycle to ensure no recurring overheat.