F25 displayed, furnace shuts down with flue/air fault indication

Safety first: Turn off power and gas. Risk of carbon monoxide if flue issues are ignored.

• Step 1 – Inspect terminal location: Ensure the flue terminal meets clearance requirements from windows, corners, and other openings. If too close, a professional must relocate it.
• Step 2 – Check for recirculation signs: Look for soot deposits around the terminal or melted plastic on the intake. This indicates exhaust being drawn back in.
• Step 3 – Examine flue integrity: Inspect accessible flue sections for disconnections or damage. Ensure the inner and outer pipes of the concentric flue are correctly assembled.
• Step 4 – Technician combustion test: A technician should perform a combustion analysis and verify flue integrity, correcting installation faults as needed.