Boiler
F09
The F09 error code on a Viessmann boiler indicates a fault with the ignition process — specifically, the boiler has failed to ignite or has lost its flame unexpectedly. This is a moderately serious fault that will leave your boiler locked out and your heating or hot water off until it is resolved.
What does this error mean?
F09 signals that the boiler’s control board detected either a failure to establish a flame during start-up, or an unexpected loss of flame during operation. The boiler shuts itself down as a safety precaution to prevent unburned gas accumulating. Viessmann Vitodens models (including the 050-W, 100-W, and 200-W) commonly display this as a lockout fault, meaning the boiler will not restart automatically.
Est. repair cost
£150–£250 for electrodes or flame sensor replacement; £250–£450 for gas valve or PCB replacement (parts and labour, UK 2024 rates)
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Reset the boiler: Press and hold the reset button (usually marked with a flame or ‘R’ symbol) for 3–5 seconds. The boiler will attempt to re-ignite. If it fires up and runs normally, monitor it over the next 24 hours.
Step 2 — Check your gas supply: Ensure other gas appliances (hob, gas fire) are working. If there is no gas elsewhere in the property, contact your gas supplier — do not attempt to work on gas pipework yourself.
Step 3 — Inspect the condensate pipe: In cold weather, the white plastic condensate pipe (usually routed outside or into a drain) can freeze solid. Thaw it carefully with a hot water bottle or warm (not boiling) water poured along the pipe. Once cleared, reset the boiler.
Step 4 — Check the boiler’s gas pressure reading: On most Vitodens models, the installer menu shows system pressure. If the display shows low water pressure (below 1 bar), top up via the filling loop and reset. Note: low water pressure is a separate issue but can trigger lockout sequences.
Step 5 — If F09 returns after reset: Do not attempt further resets beyond three attempts. Repeated lockouts with F09 point to a faulty electrode, flame sensor, or gas valve — all of which require a Gas Safe registered engineer. Stop here and call a professional.
📞 When to call a professional
Call a Gas Safe registered engineer immediately if the boiler fails to light after one or two resets, if you smell gas at any point, or if the F09 code returns within a short period of normal operation. Tell the engineer: ‘My Viessmann boiler is showing fault code F09 — it is locking out on ignition failure.’ This helps them arrive with the correct parts, most likely an ignition electrode, ionisation probe, or gas valve.
⚖️ Repair or replace?
A Gas Safe engineer callout to diagnose and repair F09 typically costs £150–£350 depending on the faulty component — electrodes and flame sensors are at the lower end, a PCB or gas valve replacement at the higher end. If your Viessmann boiler is under 10 years old, repair is almost always the right call, as Viessmann boilers have a strong reliability record and parts are widely available. If the boiler is over 15 years old and has had repeated faults, a new A-rated condensing boiler (typically £2,000–£3,500 installed) may be more cost-effective long term.