Navien furnace will not fire, display shows E001, burner never lights

Safety first: Turn off power to the Navien furnace at the service switch and close the gas shutoff valve before opening any panels. Allow the unit to cool.

Steps to check and correct:

• Verify system pressure: Check the hydronic system pressure gauge on the Navien or on the external manifold. For most Navien hydronic furnaces, cold pressure should typically be around 12–20 psi (follow the installation manual for your model). If pressure is low, use the system fill valve to add water until it reaches the recommended range. Do not overfill.
• Bleed air from the system: Air pockets can trigger low-flow/low-pressure faults. Use the automatic air vent on the Navien unit (if equipped) and any manual bleeder valves on radiators or manifolds to purge air. Run the system circulator and re-bleed until air noise is minimal.
• Clean the inlet strainer: Locate the cold-water or system return inlet strainer on the bottom piping of the Navien furnace. Close the isolation valves, remove the strainer cap, pull out the mesh screen, and rinse debris under running water. Reinstall and open valves slowly while checking for leaks.
• Inspect the water pressure sensor: The pressure sensor is typically mounted on the hydronic manifold inside the unit. Check its wiring harness for loose or corroded connections. If the sensor body is leaking or corroded, it should be replaced with a Navien OEM pressure sensor.
• Reset the unit: Restore power and gas, then press the reset button on the control panel. If E001 returns after pressure and flow are confirmed normal, the pressure sensor or control board may be faulty and a qualified technician should test and replace as needed.