Status LED flashes four times repeatedly, furnace starts but shuts down shortly after ignition

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

Steps to check:

• Replace or clean air filter: Locate the return air filter (often in the blower compartment or return grille). Replace with a properly sized filter. A clogged filter is the most common cause of high limit trips.
• Check 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 if accessible.
• Verify blower operation: With the thermostat set to FAN ON, confirm the indoor blower motor runs smoothly. Listen for squealing or grinding that may indicate a failing motor or blower wheel.
• Inspect high limit switch: Locate the high limit switch on the furnace heat exchanger panel. Check for loose wiring connections. Do not bypass the switch; it is a safety device.
• Reset and test: After correcting airflow issues, restore power and call for heat. Observe if the furnace runs a full cycle without tripping.

When to call a professional: If the blower does not run, if the high limit trips repeatedly with a clean filter and open ducts, or if you suspect a cracked heat exchanger, contact a licensed HVAC technician.