Furnace shows F6, fan may run but burner does not light, or unit locks out shortly after start attempt

Safety first: Turn off power and gas. Flue system work must comply with local regulations; if in doubt, call a qualified technician.

• Inspect flue and air intake: From the outside, check that the concentric flue terminal is not blocked by debris, ice, nests, or vegetation. Clear any obstructions carefully.
• Check pressure tubes: Open the appliance and locate the silicone tubes between the fan and the air pressure switch. Ensure they are securely connected, not kinked, and free of water or condensate. Replace if brittle or cracked.
• Test fan operation: With the casing in place and power restored, observe whether the fan starts when there is a heat demand. Unusual noises or failure to start indicate a failing fan motor.
• Pressure switch test (qualified personnel): A technician can test the air pressure switch with a manometer or by measuring continuity across the contacts when the fan runs. If the switch does not change state at the specified pressure, it should be replaced with a genuine Saunier Duval part.
• Do not bypass: Never bridge or bypass the air pressure switch; it is a critical safety device preventing operation with a blocked flue.