Furnace does not start at all, no lights or display, completely dead

Safety first: Before working on electrical components, isolate the appliance from the mains at the consumer unit.

• Check external supply: Verify that other electrical devices on the same circuit work. Reset any tripped breakers or replace blown fuses with the correct rating.
• Inspect appliance switch: Ensure the furnace’s main power switch is in the ON position and not damaged.
• Check power terminal block: Remove the front cover and inspect the mains terminal block for loose wires or signs of overheating. Tighten connections if safe and competent to do so.
• Internal fuse: Some Saunier Duval control boards have an internal fuse. A qualified technician should check and replace it if blown, and investigate the cause.
• PCB damage: If there is visible burning or the board fuse repeatedly blows, the main PCB likely needs replacement by an authorized service engineer.