Boiler shows E6 and shuts down, often accompanied by gurgling sounds in piping

Safety first: Turn off power before opening any hydronic components. Hot water can cause burns.

• Verify valves are open: Confirm that all service valves around the boiler (supply, return, and bypass) are fully open. A partially closed valve can prevent the flow switch from closing.
• Bleed air: Use the boiler's built-in air vent and any manual vents on radiators to purge trapped air. On Utica condensing boilers, check the automatic air vent on the top of the heat exchanger manifold; ensure its cap is slightly open to vent air.
• Inspect flow switch: Locate the flow switch or flow sensor on the boiler's hydronic manifold. Remove its plug and check for debris if it is a paddle or turbine type (follow Utica instructions). Clean gently and reinstall.
• Check circulator pump: Ensure the system circulator is running when there is a call for heat. If the pump is hot but not circulating, it may be seized or air-locked.
• Reset and test: Restore power, reset the boiler, and observe if the E6 clears and the boiler fires with stable flow.