Display shows F4 and spa will not heat; sometimes pumps still operate.

Safety first: Disconnect power at the main breaker. Verify with a non-contact voltage tester before touching wiring.

• Inspect heater terminals: Remove the equipment cover and visually inspect the heater terminals on the heater tube. Tighten any loose nuts and look for signs of burning or melting.
• Check for obvious board damage: Look at the Aquavia control board for darkened areas or burned relay contacts near the heater output.
• Test continuity (advanced): With power off and wires removed from the heater, use a multimeter to check resistance across the heater element. An open circuit indicates a failed heater that must be replaced.
• Replace faulty component: If the heater is open, replace the heater assembly. If the heater tests good but F4 persists, the control board/relay is likely defective and the control pack should be repaired or replaced by a qualified technician.