Furnace locks out, LED shows code 13 (limit circuit lockout). Blower may run continuously, burners off.

Safety first: Turn off power and gas supply before working inside the furnace. A limit or rollout trip can indicate overheating or unsafe combustion.

• Check air filter: Remove the furnace filter from the return air cabinet or filter rack. If it is dirty or clogged, replace it with a properly sized filter of the correct MERV rating.
• Inspect supply and return vents: Make sure supply registers and return grilles are not blocked by furniture, rugs, or closed dampers.
• Reset manual rollout/aux limits: With power off, locate rollout switches near the burners and heat exchanger on the Carrier furnace. Some have a small red reset button. If tripped, press firmly until it clicks. If a rollout switch was tripped, do not repeatedly reset it without finding the cause (cracked heat exchanger, blocked flue, or burner issues).
• Check wiring to limit circuit: Inspect wires from the control board to the primary limit and rollout switches for loose connections or burned insulation.
• Restore power and test: Turn power and gas back on, call for heat, and observe operation. If the code returns, the furnace may be overheating due to blower or heat exchanger problems.

Persistent limit lockouts require professional diagnosis to check blower performance, gas input, and heat exchanger condition.