Navien furnace locks out with E002, unit attempts to start but shuts down quickly

Safety: Turn off electrical power and close the gas valve before servicing. Allow the furnace to cool.

• Check system water level: Inspect the system pressure gauge and any connected expansion tank. If pressure is low, add water through the fill valve until the recommended pressure is reached. Check for visible leaks in piping, manifolds, and around the Navien cabinet.
• Bleed air from the primary loop: Use the built-in air vent and any purge valves on the primary loop to remove trapped air. Run the circulator pump while purging to move air toward vents.
• Inspect the flow switch: Locate the flow switch/flow sensor assembly on the hydronic inlet line inside the Navien. Disconnect power, remove the sensor, and check for scale or debris obstructing the paddle or turbine. Clean gently with a soft brush and reinstall. Ensure the wiring connector is fully seated.
• Flush the heat exchanger: If the unit is older or in a hard-water area, internal restriction can reduce flow. Isolate the furnace using service valves and connect a pump and hoses to the service ports. Circulate a manufacturer-approved descaling solution through the primary heat exchanger for the recommended time, then flush thoroughly with clean water.
• Restart and monitor: Restore power and gas, reset the unit, and observe operation. If E002 persists, the flow switch or main PCB may require replacement by a qualified Navien technician.