Burners ignite then shut off quickly; LED flashes 4 times repeatedly on Armstrong Air furnace board.

Safety: Turn off power before removing blower doors or working near wiring. Components may be hot immediately after operation.

Steps to check:

• Replace or clean air filter: Locate the return air filter (in the furnace cabinet or return grille). If it is dirty or clogged, replace it with the correct size and MERV rating recommended by Armstrong Air.
• Check registers and ducts: Ensure all supply and return registers are open and not blocked by furniture or rugs. Inspect visible ductwork for crushed or disconnected sections.
• Inspect blower operation: With the call for heat active, verify the indoor blower starts on time and reaches full speed. Listen for humming without rotation (possible bad capacitor or motor). If the blower is not running or is weak, test the blower capacitor and motor windings.
• Examine high limit switch: Locate the high limit switch on the heat exchanger panel. Check for loose wiring connections. After the furnace cools, the switch should reset automatically. If it trips repeatedly even with good airflow, test it with a meter and replace if out of specification.
• Check for undersized ductwork: If airflow restrictions are not obvious, the duct system may be undersized or poorly designed. This requires evaluation by an HVAC professional.