Status LED flashes code 13 (one short, three long). Furnace may lock out or shut down early in the cycle.

Safety first: This code can indicate overheating or flame rollout, which is a serious safety hazard. Turn off gas and power before inspection. Do not reset or bypass rollout switches without correcting the cause.

• Check and replace air filter: Remove the blower door, locate the return air filter, and replace if dirty or clogged. A restricted filter is a common cause of high limit trips.
• Inspect supply and return vents: Ensure all registers and returns are open and not blocked by furniture, rugs, or debris.
• Check blower operation: With a call for heat, verify the indoor blower starts on time and runs at normal speed. If the blower is slow or not running, suspect a failed blower motor or capacitor.
• Inspect limit and rollout switches: With power off, locate the primary limit (on the heat exchanger panel) and rollout switches (near burners). Check for manual reset buttons on rollout switches and press gently if tripped. If a rollout switch trips repeatedly, stop using the furnace and call a professional.
• Look for blocked heat exchanger or flue: Visually inspect for heavy rust, collapsed heat exchanger cells, or obstructions in the vent connector. Do not operate the furnace if you suspect a cracked or blocked heat exchanger.
• If limits are open electrically: Use a multimeter (power off) to check continuity across the limit and rollout switches. Replace any that are open and do not reset.