Signs your boiler may be unsafe — when to turn it off
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →⚠️ If you see a yellow or orange flame, smell gas, or feel unwell near the boiler — switch it off and call a Gas Safe engineer immediately.
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Most likely cause & what to check
Yellow or orange flame instead of blue: your boiler should burn with a steady blue flame. A yellow or orange flame means incomplete combustion — this produces carbon monoxide. Stop using the boiler immediately.
Soot or scorching around the boiler casing: black marks around the case suggest the boiler has been "spilling" flue gases into the room rather than venting them outside. This is an immediate carbon monoxide risk.
The pilot light keeps going out repeatedly: while not always dangerous on its own, a repeatedly failing pilot can indicate a gas supply problem, a failing thermocouple, or a draught issue with the flue.
Unusual smells when the boiler is running: a faint gas smell when the boiler first lights is sometimes normal (unburnt gas before ignition) but a persistent smell is not. A "hot electrics" smell suggests an internal component is overheating.
The boiler keeps cutting out on a safety lockout: modern boilers lock out to prevent unsafe operation. Repeated lockouts on the same fault are the boiler telling you it cannot operate safely — do not keep resetting it.
Your CO alarm keeps triggering: treat this as a real alert, not a nuisance. Get everyone out and call the gas emergency line.
The flue outside is blocked or damaged: the flue terminal should be unobstructed and not blocked by nesting birds, leaves, or impact damage. A blocked flue causes CO to spill back into the property.
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