Furnace starts but shuts down quickly; Miller board LED flashes 4 times repeatedly.

Safety: Turn off power before removing blower doors. High temperatures can cause burns; allow the furnace to cool.

• Check air filter: Locate the return air filter in the Miller furnace or return grille. Remove and inspect. If dirty or clogged, replace with the correct size and airflow rating. Never run the furnace without a filter for extended periods.
• Inspect registers and ducts: Ensure supply registers and return grilles are open and not blocked by furniture or rugs. Check for crushed or disconnected flexible ducts, especially in manufactured homes where Miller furnaces are common.
• Verify blower operation: Call for heat and observe if the indoor blower starts after the burners ignite. If the blower does not start, or cycles on and off rapidly, test the blower motor capacitor and motor windings. Replace the blower motor or capacitor as needed.
• Check high limit switch: Locate the high limit (usually mounted on the heat exchanger panel). With power off, check for continuity across the switch when the furnace is cool. If it remains open when cool, replace the high limit switch with the correct Miller part. If it repeatedly trips, address airflow or heat exchanger issues before resetting.