Furnace locked out, no heating demand served, display shows F0 and burner does not attempt to ignite

Safety first: Turn off the main power switch to the furnace and close the gas isolation valve before opening any panels. Wait at least 5–10 minutes for internal components to cool.

• Reset the appliance: Restore power and follow the Saunier Duval user manual procedure to reset lockout (usually pressing and holding the reset button on the control panel for a few seconds). Observe if the code clears and the unit attempts ignition.
• Check for prior errors: Many Saunier Duval furnaces store a history of faults. If accessible, view the last error codes to see if repeated ignition or flame faults preceded F0. This can indicate a genuine safety issue rather than a PCB failure.
• Inspect safety chain: With power off, visually inspect the overheat thermostat, flue thermostat, air pressure switch, and their wiring to the main PCB. Look for loose spade connectors, burnt terminals, or damaged insulation. Reseat connectors firmly.
• Check supply voltage: Confirm that the furnace is receiving stable mains voltage within the specified range using a multimeter (only if you are trained and competent). Voltage outside spec can cause control board lockouts.
• If F0 persists: Do not bypass any safety devices. A persistent F0 after reset and wiring checks usually indicates a defective control board or a safety device that is intermittently opening. Contact an authorized Saunier Duval service technician for PCB diagnostics and replacement.