Navien furnace shows E016 and shuts down during operation, often after running for a while

Safety: Turn off power and gas. Allow the unit to cool completely before touching internal components.

• Check system valves: Verify that all supply and return isolation valves near the Navien furnace are fully open. Ensure zone valves or manifold valves are not closed.
• Inspect circulator pump operation: With the front cover removed and power restored briefly, call for heat and listen/feel for the internal primary circulator pump. If it hums but does not move water (no vibration in pipes), it may be seized. Turn power back off and consider replacing the circulator pump.
• Clean system strainer: A clogged strainer on the return line can reduce flow. Close isolation valves, remove the strainer cap, clean the mesh screen, and reinstall.
• Descale the primary heat exchanger: Connect a flushing pump and hoses to the service valves. Circulate a Navien-approved descaling solution through the primary heat exchanger for the recommended time, then flush thoroughly with clean water. This restores flow and heat transfer.
• Check high-limit sensor: Locate the high-limit temperature sensor on the heat exchanger. Inspect wiring and mounting. If the sensor is loose, remount with proper contact. If it is cracked or out of spec (requires meter), replace it.
• Reset and monitor: Restore power and gas, reset the unit, and monitor supply and return temperatures. If E016 recurs with good flow, have a technician test the high-limit sensor and control board.