Boiler
F12
The F12 error code on an Ideal boiler indicates a flame detection fault — the boiler is failing to detect a stable flame after attempting to ignite, which means it will lock out and stop producing heat or hot water. This is a moderately serious fault that almost always requires a Gas Safe registered engineer.
What does this error mean?
When your Ideal boiler displays F12, the boiler’s control board has attempted to light the burner but the flame sensor (also called the ionisation probe) has not confirmed a stable flame within the expected timeframe. This causes the boiler to enter a safety lockout to prevent unburned gas from accumulating. It is the boiler doing exactly what it should — protecting your home — but the underlying cause needs diagnosing and fixing before heating will resume.
Est. repair cost
£80–£200 for probe/electrode repair by a Gas Safe engineer; new boiler £1,800–£3,000 installed if replacement is needed
🔍 Likely causes
🛠️ Step-by-step fix
Check your other gas appliances (hob, gas fire) are working normally — if not, call your gas supplier immediately as there may be a supply issue. Do not attempt to use the boiler.
Check whether your condensate pipe (the small plastic pipe that exits your home, usually to an outside drain) is frozen or blocked — in cold weather, thawing it with warm (not boiling) water and insulating it is a safe DIY fix.
Press the reset button on your Ideal boiler (usually a button with a flame symbol or labelled ‘Reset’) and wait 5–10 minutes. If the boiler fires up and runs normally, monitor it closely over the next 24 hours.
If the fault returns after one or two resets, do not continue resetting — repeated lockouts can indicate a worsening fault and further resets without a fix are not a solution.
At this point, stop all DIY intervention. The remaining causes (probe, electrode, gas valve, PCB) all involve internal gas components and must only be inspected and repaired by a Gas Safe registered engineer.
📞 When to call a professional
Call a Gas Safe registered engineer if the F12 fault returns after a reset, or if you smell gas at any point — in which case call the National Gas Emergency line immediately on 0800 111 999. When booking an engineer, tell them your boiler model (found on the front panel), that you have an F12 fault code, and approximately how long it has been occurring — this helps them bring the right parts.
⚖️ Repair or replace?
A typical F12 repair — cleaning or replacing the ionisation probe or ignition electrode — costs between £80 and £200 including labour, making repair very worthwhile for a boiler under 10 years old. If your boiler is over 10–12 years old and this is one of several recent faults, a new A-rated boiler (typically £1,800–£3,000 installed) is likely more cost-effective in the long run given improved efficiency and warranty coverage. For a well-maintained boiler under a decade old, repair is almost always the right call.