F3 displayed, spa may heat erratically or not at all

Safety first: Disconnect power at the breaker before accessing the control pack or heater area.

• 1. Power reset: Turn breaker OFF for 2–3 minutes, then ON. If F3 reappears, continue.
• 2. Locate temperature sensor:
  • Open the equipment bay and find the heater tube and control pack.
  • Identify the water temperature sensor (usually a small probe inserted into a dry well on the heater or plumbed tee, with a cable going to the control board).
• 3. Check sensor seating and cable:
  • Ensure the probe is fully inserted into its well or fitting.
  • Inspect the cable for cuts, pinches, or rodent damage.
  • Unplug the sensor from the control board and re-seat the connector firmly.
• 4. Compare displayed temperature:
  • Restore power and compare the displayed temperature to actual water temperature (use a separate thermometer).
  • If the reading is wildly inaccurate or F3 persists, the sensor is likely defective.
• 5. Replace sensor if needed:
  • Order a compatible Wellis water temperature sensor matched to your control system.
  • With power OFF, remove the old sensor and install the new one in the same location and connector.
• 6. If a new sensor does not resolve F3: The main control board’s sensor input circuit may be faulty. Have a professional test and, if necessary, replace the control pack.