Furnace shows flue gas recirculation or combustion air fault, may shut down with safety lockout

Safety: Shut down the boiler and do not operate it if flue gas recirculation is suspected. Risk of carbon monoxide.

• Inspect terminal: From outside, check the Viessmann concentric flue terminal for obstructions (snow, ice, leaves, nests). Clear any blockage carefully.
• Check flue joints: Visually inspect accessible flue sections for dislodged or separated joints. Ensure all locking bands and seals are intact.
• Check air intake: On systems with separate air intake, verify that the intake is not blocked or drawing from a contaminated area.
• Wind effects: If the fault occurs only during strong winds, a terminal deflector or relocation may be required; consult a Viessmann-trained installer.
• Do not modify flue yourself: Any flue reconfiguration must be done by a qualified technician following Viessmann flue design guidelines.