Unit stops and displays E04; fan may continue briefly then shut down.

Safety first: Turn off the heater and pool pump before opening any valves or filters.

• Verify pool pump operation: Ensure the circulation pump is running and primed. Check for air in the pump basket and re-prime if necessary.
• Clean filtration system: Empty skimmer and pump baskets. Backwash or clean the pool filter to restore normal pressure and flow.
• Check valves and bypass: Confirm that inlet and outlet valves to the Hewalex heater are fully open. If a bypass is installed, adjust it so that a strong flow passes through the heater (follow Hewalex hydraulic diagrams).
• Inspect flow switch: Locate the water flow switch or flow sensor on the heater’s water circuit. Check for kinks or blockages in the piping. Gently tap the switch body to free a stuck paddle (if paddle type).
• Reset and test: Restore power and restart the heater. If E04 persists with good flow, the flow switch or sensor may be defective and require replacement.