BoilerFaultCodesYour boiler has detected that waste gases cannot safely leave the property through the flue pipe, causing the system to shut down for safety.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Blue light Lockout reset button
What causes this fault?
This fault occurs when the boiler's internal sensors detect that the exhaust gases are not being cleared from the system effectively. The most common reasons include a physical obstruction in the external flue pipe, such as a bird's nest or debris, or a mechanical failure of the fan that pushes these gases out. Because these gases are dangerous, the boiler will immediately stop working as a safety precaution.
This issue is slightly more frequent in winter as high winds can cause pressure fluctuations in the flue, or heavy snowfall can physically block the external terminal.
DIY fix possible
Check the external flue pipe for any visible obstructions like bird nests, debris, or leavesIn freezing weather, check the end of the flue for ice buildup and carefully clear itPress and hold the blue reset button for 3 to 5 seconds to see if the boiler restarts
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like Flue blockage — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.
Worcester Bosch Greenstar 12i System is discontinued — time for a new one?
Parts for discontinued models can be hard to find and expensive. A new A-rated boiler could save you money on both repairs and energy bills.
Affiliate links — we may earn a commission at no extra cost to you.