The furnace starts but shuts down before reaching temperature; the LED indicates an auxiliary limit is open; the blower may continue to run.

Safety: Turn off power before removing blower or coil access panels.

Steps:

• Check filter and ducts: Replace dirty filters and ensure return air pathways are not blocked.
• Inspect evaporator coil: If you have AC, remove the coil access panel above the furnace (if accessible). Look for a matted, dirty coil face. Clean with an approved coil cleaner or have a professional clean it.
• Check blower wheel: Remove the blower assembly and inspect the blower wheel for dust buildup that can reduce airflow. Clean with a brush and vacuum.
• Test auxiliary limit: With the furnace cool and power off, test the auxiliary limit switch for continuity. Replace if open when cool.

If the auxiliary limit continues to open after airflow issues are corrected, a technician should evaluate blower sizing, motor performance, and duct design.