Boiler
E11
The E11 error code on a Viessmann boiler indicates a flame supervision fault — the boiler is either failing to ignite or losing its flame shortly after lighting, which means no heating or hot water until it’s resolved.
What does this error mean?
E11 signals that the boiler’s ionisation sensor — the component that confirms a flame is present and stable — is not detecting a flame as expected. This could mean the boiler attempted to fire up but the burner failed to ignite, or the flame lit briefly then cut out. Viessmann boilers are designed to lock out under this condition as a safety measure to prevent unburned gas building up.
Est. repair cost
£80–£180 for sensor or electrode replacement; £250–£500+ if gas valve or PCB is at fault. All work must be carried out by a Gas Safe registered engineer.
🔍 Likely causes
🛠️ Step-by-step fix
Check your gas supply first: turn on a gas hob or another gas appliance to confirm gas is reaching your home. If there’s no gas, contact your supplier — this is not a boiler fault.
Reset the boiler: press and hold the reset button (usually marked with a flame and cross symbol) for 3–5 seconds, then release. Allow up to 2 minutes for the boiler to attempt a restart.
If the error clears but returns within a few hours, repeat the reset once more — persistent E11 after two resets means a component fault that requires a Gas Safe engineer.
Check the boiler’s flue outlet (externally, if safely accessible): a blocked or bird-nested flue can disrupt combustion and cause flame loss. Do not attempt to clear it yourself — report it to an engineer.
Do NOT attempt to access the gas valve, burner, or ionisation sensor yourself. Working on internal gas components without Gas Safe registration is illegal and dangerous.
Call a Gas Safe registered engineer and quote error code E11, describing how often the boiler locks out and whether it briefly fires before cutting off — this helps them diagnose quickly.
📞 When to call a professional
If the boiler does not clear after one or two resets, or if E11 returns repeatedly, stop resetting and call a Gas Safe registered engineer immediately — repeated failed ignition attempts can cause unburned gas accumulation. Tell the engineer it’s a Viessmann E11 flame supervision fault and note whether the burner fires briefly before locking out or fails to ignite at all, as this distinguishes between an ignition electrode fault and a gas valve or sensor issue.
⚖️ Repair or replace?
Most E11 repairs — replacing an ionisation sensor (£20–£50 part) or ignition electrode (£15–£40 part) — are straightforward for an engineer and cost-effective on boilers under 10 years old. If your Viessmann is over 12–15 years old and the PCB or gas valve is at fault (£150–£400+ in parts alone), replacement with a modern A-rated boiler is worth serious consideration, particularly given current energy efficiency gains. For a boiler under a decade old in otherwise good condition, repair is almost always the right call.