Status LED flashes four times, furnace shuts down shortly after ignition attempt.

Safety First: Turn off power before accessing internal components. Overheating can damage the heat exchanger; do not repeatedly reset without finding the cause.

This code indicates the high limit switch opened due to excessive temperature in the heat exchanger compartment.

• Check air filter: Locate the furnace filter (often in the return air plenum or filter rack). If dirty or clogged, replace with the correct size and airflow rating recommended by Thermo Pride.
• Inspect supply and return vents: Make sure all registers and grilles are open and not blocked by furniture, rugs, or debris.
• Check blower operation: Set the thermostat fan to "ON" and verify the blower runs smoothly at full speed. Listen for unusual noises or slow start. If the blower does not run, suspect the blower motor, capacitor, or control board.
• Inspect high limit switch: With power off, locate the high limit switch on the heat exchanger panel. Check for loose wires or signs of overheating. Some models have manual reset limits; if tripped, press the reset button once only after correcting airflow issues.
• Check filter rack and cabinet for air leaks: Air leaks can reduce airflow across the heat exchanger. Ensure panels are properly installed and gaskets intact.

When to call a professional: If the limit continues to trip after filter replacement and airflow checks, or if you suspect a cracked heat exchanger, contact a qualified HVAC technician immediately.