Furnace control shows open auxiliary limit or vent temperature switch; unit shuts down mid-cycle.

Safety first: Turn off power and gas. Components may be hot.

Steps:

• Locate auxiliary limit switches on the blower housing, vestibule panel, or near the vent connector. These are smaller disc-type switches wired in series with the main limit.
• Inspect for loose wiring or signs of overheating.
• On condensing CTC furnaces, check the condensate trap and drain tubing for clogs. Clean the trap and flush tubing with warm water until flow is clear.
• Inspect PVC vent and intake pipes for sags holding water or obstructions at terminations.
• After clearing restrictions, reset any manual-reset auxiliary limits if present.
• If the switch opens again with proper airflow and drainage, replace the affected CTC auxiliary limit switch with the same temperature rating.