Furnace blower runs but burners will not light; LED shows 4 flashes on York control board.

Safety: Turn off power before removing panels. Components near the burners and heat exchanger can be hot.

• Check air filter: Locate the return air filter (often in the blower compartment or return grille). If it is dirty or collapsed, replace it with the correct size and airflow rating. Never run without a filter for more than brief testing.
• Inspect registers and ducts: Make sure supply registers and return grilles are open and not blocked by furniture, rugs, or debris. Check for crushed or disconnected flex duct if accessible.
• Locate high limit switch: On York furnaces, the main limit is usually mounted on the heat exchanger panel or in the supply air plenum. Inspect wiring for loose connections or signs of overheating.
• Reset and test: Some limit switches are manual reset (small red button). If tripped, press gently until it clicks. Restore power and call for heat. Monitor operation; if the limit trips again, there is an underlying airflow or overfire issue.
• When to call a pro: Repeated limit trips can indicate a cracked heat exchanger, incorrect gas input, or undersized ductwork. These require professional diagnosis.