E03 appears and the spa stops heating; sometimes accompanied by very hot water or rapid temperature rise.

Safety first: If the water feels excessively hot, do not enter the spa. Unplug the unit immediately and allow it to cool with the cover open.

• Allow cooling: Leave the cover partially open and let the water cool below 35°C (95°F) before further checks.
• Check filter and flow:
  • Remove and clean or replace the Avenli filter cartridge.
  • Ensure all inlet/outlet fittings are fully open and not kinked.
  • Run filtration only (no heat) for 10–15 minutes to confirm good water movement.
• Inspect heater area:
  • With power disconnected, access the heater tube assembly and verify hoses are not kinked or collapsed.
  • Check for scale buildup or debris at the heater inlet/outlet if accessible.
• Reset overheat: Some Avenli control boxes have an internal manual reset on the high-limit sensor. If present, press the small red or black reset button after the unit has cooled.
• Persistent E03:
  • If E03 returns quickly with normal water temperature, the high-limit sensor or main control PCB (heater relay) may be faulty.
  • Replace the high-limit sensor first; if the heater continues to run when it should be off, replace the control box assembly.