Boiler
F12
The F12 error code on a Ferroli boiler indicates a flame detection failure — the boiler is attempting to ignite but cannot confirm a stable flame, and has locked out for safety. This is a serious fault that will leave you without heating or hot water until it is resolved.
What does this error mean?
Your Ferroli boiler uses a flame sensor (ionisation probe) to confirm that the burner has lit successfully after ignition. When the control board cannot detect a flame signal within the expected timeframe, it triggers the F12 lockout and shuts the boiler down to prevent unburned gas from accumulating. This is a built-in safety mechanism, not a glitch — the boiler is doing exactly what it should by refusing to keep firing without confirmed ignition.
Est. repair cost
£150–£350 for a Gas Safe engineer call-out, diagnosis and parts (ionisation probe, electrodes, or gas valve). New boiler if replacement needed: £1,800–£3,000 installed.
🔍 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. Wait 60 seconds and see if the boiler fires up normally. A one-off lockout after a power cut or gas interruption is common and a reset may clear it.
Step 2 — Check your gas supply: Turn on a gas hob or check another gas appliance. If nothing is working, call your gas supplier (National Gas Emergency: 0800 111 999) — do not attempt to reset the boiler if you suspect a gas supply issue.
Step 3 — Check for a gas supply interruption on National Grid’s website or contact your supplier to rule out a local outage in your area.
Step 4 — If the boiler locks out again immediately after resetting, stop here. Do not attempt more than two resets. Repeated lockouts mean there is an underlying fault that requires a Gas Safe registered engineer — further DIY is not safe or legal for gas components.
Step 5 — Call a Gas Safe registered engineer and report the F12 code. Tell them the boiler model (found on the front panel or data plate), how many times it has locked out, and whether other gas appliances are working. This helps them diagnose the fault before arriving.
📞 When to call a professional
Call a Gas Safe registered engineer if the boiler locks out again after one or two resets, or immediately if you smell gas at any point. Any work beyond a basic reset — including cleaning the ionisation probe, replacing electrodes, testing the gas valve, or inspecting the flue — must be carried out by a qualified Gas Safe engineer. Tell them you have an F12 flame failure lockout and give them your Ferroli model number so they can bring likely replacement parts.
⚖️ Repair or replace?
A typical F12 repair — covering engineer call-out, diagnosis, and parts such as an ionisation probe or ignition electrodes — costs between £150 and £350 in the UK depending on your region and the specific fault. If your Ferroli boiler is over 10–12 years old and this is not the first significant fault, repair costs can quickly approach the value of the boiler itself; in that case, a new boiler (£1,800–£3,000 installed) is often the more cost-effective long-term choice. For boilers under 8 years old with no history of major faults, repair is almost always the right call.