Boiler shows E005 and shuts down during operation, often with high temperature readings

Safety first: Allow the boiler to cool before touching piping. Turn off power before working on pumps or sensors.

• Step 1 – Check system valves: Verify all supply and return isolation valves near the boiler are fully open. Partially closed valves can cause rapid overheating.
• Step 2 – Confirm pump operation: Call for heat and feel the primary pump housing. It should vibrate slightly and get warm. If silent and cold, check the pump's electrical connector. A non‑running pump may require replacement.
• Step 3 – Bleed air: Air in the system can stop flow. Use air vents on the boiler and system to purge air. Watch the pressure gauge and maintain proper system pressure while bleeding.
• Step 4 – Clean system strainer: Shut off isolation valves and remove the system strainer or Y‑strainer. Clean the mesh screen thoroughly and reinstall.
• Step 5 – Check high limit sensor: Inspect the high limit temperature sensor (thermistor) on the heat exchanger for secure mounting and wiring. A loose or damaged sensor can misread temperature.

If E005 recurs after restoring flow and bleeding air, the heat exchanger may be scaled or restricted and should be descaled or inspected by a Navien‑qualified technician.