Burner lockout, furnace will not start, red fault LED on control, no flame established

Safety first: Turn off electrical power to the furnace at the service switch and close the external gas shutoff valve before opening the cabinet. Allow the heat exchanger to cool.

• Check gas supply: Verify the external gas shutoff is open and that other gas appliances in the home work. If there is no gas supply, contact your gas provider.
• Inspect ionization (flame) electrode: Remove the burner cover on the Viessmann unit. Locate the ionization electrode near the burner. Clean it gently with fine emery cloth, ensuring no soot or oxidation remains. Check that the ceramic insulator is not cracked and that the electrode tip is 3–4 mm in the flame path (refer to the Viessmann service manual for exact gap).
• Inspect ignition electrode and cable: Check for cracks in the ceramic, carbon tracking, or damaged insulation on the high-voltage cable. Reseat both ends of the cable on the ignition transformer and electrode. Replace the ignition electrode assembly if damaged.
• Check gas valve and wiring: Confirm the gas valve coil connectors are firmly seated on the Viessmann gas valve. Look for loose or corroded spade terminals. Do not attempt to open or adjust the gas valve yourself; if the valve does not click or open during ignition, call a qualified technician.
• Reset the boiler: Restore power and gas, then press the reset button on the Viessmann control panel once. If the F1 fault returns repeatedly, stop resetting and schedule professional service.