Boiler locks out with F1 on display, burner does not ignite

Safety first: Turn off the boiler at the main switch and isolate the gas supply before removing any covers. Only a Gas Safe / licensed gas technician should work on gas-carrying components.

• Power reset: Turn the boiler off for 30 seconds, then back on. Observe if F1 reappears during ignition.
• Check gas supply: Ensure the gas isolation valve to the boiler is open and other gas appliances in the property work correctly.
• Inspect ignition electrode: With power and gas isolated, remove the burner cover. Check the Viessmann ignition electrode for cracks, heavy carbon deposits, or damaged insulation. Clean gently with fine emery cloth; replace if worn.
• Check flame sensor (ionisation electrode): Ensure correct position in the flame, clean light deposits, and verify the cable is firmly connected to the control board.
• Grounding: Confirm the boiler chassis is properly earthed; loose earth can prevent flame detection.
• If F1 persists: Do not repeatedly reset. Contact a qualified technician to test ignition voltage, gas valve pressure, and flame rectification current using Viessmann service instructions.