Unit displays E003 and shuts down; burner may start and then stop, or may not start at all.

Safety: Turn OFF power and close the gas shutoff valve before removing the cover. Do not work on live gas components.

Steps:

• 1. Reset and observe: Power cycle the unit and run hot water. Note if the burner lights and then goes out before E003 appears. This suggests flame loss rather than no ignition.
• 2. Check venting and combustion air: Inspect intake and exhaust terminations for blockages. Ensure vent lengths and elbows comply with Navien installation guidelines; excessive length can cause poor combustion.
• 3. Clean flame rod: Remove and clean the flame rod with fine abrasive pad to remove oxidation. Ensure the mounting bracket and screws provide a solid ground connection to the burner.
• 4. Inspect burner surface: Look for heavy dust, lint, or corrosion on the burner. Gently vacuum or brush off loose debris without damaging the burner mesh.
• 5. Check wiring: Confirm the flame rod wire and ground connections are tight at both the sensor and the main PCB.

If E003 continues after cleaning and vent checks, a technician should verify gas pressure, combustion settings, and possibly replace the flame rod or main PCB.