Furnace lights but shuts off quickly; LED shows 4 flashes; burners may cycle on and off repeatedly.

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

Steps:

• Check air filter: Locate the return air filter (in the furnace or return grille). If dirty or clogged, replace with the correct size and airflow rating. Never run the furnace without a filter for extended periods.
• Inspect supply and return vents: Ensure all registers and grilles are open and not blocked by furniture, rugs, or debris. Check for crushed or disconnected ductwork in accessible areas.
• Verify blower operation: Call for heat and observe the blower start-up. If the blower does not start or is weak, check the blower motor capacitor and motor for overheating or noise.
• Check high limit switch: With power off, locate the high limit switch on the heat exchanger panel. Inspect wiring connections. If the switch is manually resettable, press the reset button (if present). Use a meter to confirm continuity when the furnace is cool. Replace if it remains open when cool.

If airflow is good and the high limit continues to open, the furnace may be overfiring or have a heat exchanger or duct design issue. Contact a qualified HVAC technician for further evaluation.