Navien furnace shows E003 and will not ignite, fan may run briefly then stop

Critical safety warning: This fault involves gas and ignition. If you smell gas, do not attempt repair. Evacuate the area and contact your gas supplier or emergency services. For all work on gas components, a licensed technician is strongly recommended.

• Verify gas supply: Confirm that the manual gas shutoff valve to the Navien furnace is fully open. Check that other gas appliances in the home are working. If none are working, contact the gas utility.
• Check venting and air intake: Inspect the PVC vent and intake pipes for blockages (snow, leaves, nests). Clear any obstructions and ensure terminations meet Navien clearance requirements.
• Inspect flame rod and igniter: With power off and gas shut, remove the burner access cover. Locate the flame rod and ignition electrode near the burner. Remove them carefully and clean the flame rod with fine emery cloth to remove oxidation. Check for cracks in the ceramic insulators. Replace the flame rod or igniter if damaged.
• Check wiring connections: Ensure the flame rod and igniter harnesses are firmly connected to the main PCB and not pinched or burned.
• Attempt reset: Restore power and gas, then press reset. Observe through the sight glass: the fan should start, then ignition spark, then flame. If there is no flame despite gas supply, the gas valve or control board may be faulty and should be tested and replaced by a qualified Navien service technician.