Furnace displays F5, burner shuts down, may occur during operation rather than at startup

Safety first: Isolate power and gas before opening the combustion chamber. Only qualified personnel should work on combustion components.

• Clean flame sensor: On many Saunier Duval furnaces, the ionization (flame) electrode is combined with the ignition electrode. Carefully remove and clean the sensing surface with fine abrasive pad. Avoid damaging the ceramic insulator.
• Check electrode position: Ensure the electrode is correctly positioned in the flame as per the service manual. Incorrect positioning can cause intermittent flame detection.
• Inspect wiring: Examine the ionization cable from the electrode to the PCB for cracks, burns, or loose connections. Replace if insulation is damaged.
• Check gas stability: A gas engineer should verify that inlet gas pressure is stable and that burner pressure and CO₂/O₂ levels are within Saunier Duval specifications. Fluctuating pressure can cause flame dropout.
• Control board test: If the flame signal is strong but the PCB still reports loss of flame, the main control board may be defective and require replacement by an authorized technician.