Navien furnace displays E004, unit shuts down shortly after ignition attempt

Safety: Turn off power and close the gas valve before opening the unit. Work on gas and combustion components should be performed by qualified personnel.

• Check vent and intake: Inspect the entire length of the exhaust and intake pipes for sags, water accumulation, or obstructions. Clear any blockages and correct improper slopes that allow condensate to pool.
• Inspect condensate drain: A backed-up condensate trap can affect combustion. Locate the condensate trap at the bottom of the unit, remove the drain hose, and check for sludge or debris. Flush the trap with clean water and ensure the drain line slopes continuously to the drain.
• Clean burner and flame rod: Remove the burner cover and visually inspect the burner surface for dust, rust, or debris. Use a soft brush or compressed air (low pressure) to clean. Remove and clean the flame rod with fine abrasive pad. Reinstall securely.
• Check gas supply stability: If other gas appliances fluctuate or go out, there may be a supply or regulator issue. A licensed technician should measure inlet and manifold gas pressures while the unit is firing.
• Reset and test: Restore power and gas, reset the unit, and monitor operation. If E004 recurs, the flame sensing circuit on the PCB or the gas valve may be defective and require professional diagnosis and replacement.