Furnace locks out with flue gas temperature error; burner stops during operation

Safety: Turn off power and gas. Do not operate the furnace with a known flue fault due to risk of carbon monoxide.

• Inspect flue termination: Outside, check the IVT flue outlet for obstructions such as ice, snow, bird nests, or debris. Clear any blockage.
• Check flue routing: Visually inspect accessible flue sections for sagging, crushed pipe, or disconnected joints. Any damage must be corrected by a qualified installer.
• Examine flue gas sensor: Locate the flue gas temperature sensor on the flue collector or outlet. Ensure it is firmly mounted and its wiring harness is intact and connected to the IVT control board.
• Reset and test: After clearing obstructions and checking connections, reset the furnace and monitor operation. If the error recurs, combustion settings or the sensor itself may be faulty.

Flue gas temperature and combustion tuning should only be adjusted by a certified technician with proper instruments.