Furnace shuts down on high temperature; limit switch open error.

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

Steps:

• Check and replace the furnace air filter with the correct size and MERV rating recommended by CTC. A severely clogged filter is a common cause of limit trips.
• Ensure all supply registers and return grilles in the home are open and unobstructed.
• Observe the circulating blower operation. If the blower does not start or is weak, inspect the CTC blower motor and capacitor (if PSC type) for failure.
• Locate the CTC high limit switch on the heat exchanger panel. Inspect wiring for loose connections.
• Some limit switches are manual reset; if so, press the reset button once after the furnace has cooled. If it repeatedly trips, do not continue resetting—find and correct the root cause.
• If airflow is good and the blower is working but the limit still trips, the limit switch may be weak and require replacement with the correct temperature-rated CTC limit switch.

Warning: Chronic limit tripping can damage the heat exchanger. If overheating persists, have a professional evaluate the furnace.