Boiler locks out with E003 shortly after ignition attempt, sometimes with brief flame

Safety first: Shut off power and gas before removing the burner or gas components. Only qualified personnel should adjust gas pressures.

• Step 1 – Observe flame behavior: Reset the boiler and watch through the sight glass. If the flame lights then drops out within a few seconds, the flame sensing or gas supply is suspect.
• Step 2 – Clean flame rod and burner: With power and gas off, remove the burner assembly per Navien instructions. Clean the flame rod with fine abrasive and inspect the burner surface for rust, debris, or blocked ports. Vacuum dust and debris; do not enlarge burner holes.
• Step 3 – Inspect condensate drain: A blocked condensate trap can flood the combustion chamber and destabilize flame. Check the condensate trap and drain line for kinks, ice, or blockage. Clean the trap and ensure a proper slope to the drain.
• Step 4 – Check venting: Verify intake and exhaust PVC/CPVC/PP vent pipes are not blocked by snow, leaves, or nests. Ensure all joints are properly glued and there are no sags collecting condensate.
• Step 5 – Gas supply and valve: Have a licensed technician verify inlet and manifold gas pressures during operation. If pressure drops excessively or the valve does not modulate correctly, the gas valve assembly or regulator may need replacement.

Persistent E003 faults require professional combustion analysis and possible replacement of the flame rod, gas valve, or PCB.