Boiler displays E004 and will not start, often accompanied by fan running briefly

Safety first: Turn off power before accessing internal components. Use a stable ladder if checking outdoor vent terminations.

• Step 1 – Inspect vent terminations: Go outside and check both intake and exhaust terminations. Remove any snow, ice, leaves, or insect nests. Ensure clearances from walls and other structures meet Navien requirements.
• Step 2 – Check vent piping: Inspect visible vent runs for sags, separations, or signs of water pooling. Correct any low spots and ensure proper support and slope.
• Step 3 – Inspect air pressure switch tubing: With power off, remove the front cover. Locate the air pressure switch and its silicone tubes to the blower/venturi. Ensure tubes are not cracked, kinked, or disconnected. Replace damaged tubing and ensure tight fits.
• Step 4 – Test blower operation: Restore power and call for heat. Listen for the combustion blower starting. If the blower does not run, check its wiring harness and connector. A non‑running blower or seized motor will require replacement.
• Step 5 – Air pressure switch check: If the blower runs but E004 persists, the pressure switch may not be closing. A technician can test continuity across the switch with a multimeter during operation and replace the switch if it fails.

If venting and tubing are clear and the blower runs normally, the air pressure switch or main PCB may need professional diagnosis and replacement.