Boiler
F08
The F08 error code on a Viessmann boiler indicates a burner lockout fault — meaning the boiler tried to ignite, failed, and has shut itself down as a safety precaution. It’s a fairly common fault, but because it involves gas ignition, most fixes must be carried out by a Gas Safe registered engineer.
What does this error mean?
F08 signals that the boiler’s burner failed to light within the allowed number of attempts, triggering an automatic lockout to prevent unburned gas from building up. The boiler’s control board detected either no flame signal or an unstable flame during the ignition sequence. This is a safety lockout — the boiler will not restart on its own until the underlying cause is resolved.
Est. repair cost
£150–£300 for ignition electrode or flame sensor replacement by a Gas Safe engineer; gas valve replacement typically £250–£450
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Check your gas supply: Confirm other gas appliances (cooker, gas fire) are working. If not, contact your gas supplier — there may be a supply interruption. Do not proceed with resets until supply is confirmed.
Step 2 — Reset the boiler: Press and hold the reset button on your Viessmann boiler (usually marked with a flame symbol or ‘Reset’) for 3–5 seconds. The boiler will attempt to re-ignite. If it fires up and stays on, monitor it over the next 24 hours.
Step 3 — Repeat once if needed: If the first reset fails, wait 5 minutes and try a second reset. Attempting more than two resets in quick succession is not recommended and will not resolve a mechanical or component fault.
Step 4 — Check the flue terminal: If safe to do so, visually inspect the external flue outlet (usually on an outside wall) to ensure it isn’t blocked by debris, bird nests, or ice in cold weather. Do not touch or dismantle any flue components.
Step 5 — Stop here and call a Gas Safe engineer: If the error returns after two resets, or if you cannot identify an obvious external cause, do not attempt any further DIY. Ignition components, gas valves, and flame sensors must only be inspected and replaced by a qualified Gas Safe registered engineer.
📞 When to call a professional
Call a Gas Safe registered engineer if the F08 fault returns after two reset attempts, or immediately if you smell gas at any point — in which case, leave the property and call the National Gas Emergency Service on 0800 111 999. When booking an engineer, tell them your boiler model (e.g. Viessmann Vitodens 100-W), that you have an F08 burner lockout fault, and roughly how many times it has locked out, as this helps them diagnose the issue faster.
⚖️ Repair or replace?
A typical ignition electrode or flame sensor replacement by a Gas Safe engineer costs £150–£300 including parts and labour — a worthwhile repair if your boiler is under 10 years old. If your Viessmann boiler is over 12–15 years old and showing recurring faults, replacement with a new A-rated combi boiler (£2,000–£3,500 installed) becomes more cost-effective than ongoing repairs. Viessmann boilers are generally high quality and worth repairing if they’re within the first decade of life.