Furnace displays E04 and shuts down shortly after blower starts

Safety first: Turn off power before removing blower door or working near wiring.

• Replace or clean filter: Locate the return air filter (often in the return grille or furnace filter rack). If dirty or clogged, replace with the correct size and MERV rating recommended by Napoleon.
• Check vents and returns: Walk through the home and open all supply registers and return grilles. Remove any furniture or rugs blocking airflow.
• Inspect blower operation: With power restored and thermostat calling for heat, verify that the main blower starts and runs smoothly. Listen for unusual noises or slow start. If the blower is weak or not running, the furnace can overheat quickly.
• Locate high limit switch: The high limit is usually mounted on the heat exchanger panel inside the furnace. Check for loose wires or signs of overheating. Do not bypass the limit switch.
• When to replace: If airflow is good and the blower is operating correctly but E04 persists, the high limit switch may be weak or out of calibration and should be replaced with the correct Napoleon-rated limit switch. Persistent overheating may indicate a cracked or restricted heat exchanger and requires professional inspection.