BoilerFaultCodesYour boiler has likely overheated or detected a blockage in the chimney pipe, causing it 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: Flue overheat / Heat exchanger overheat / Flue blockage. Check: Heat exchanger blocked; Heat exchanger baffles removed and not refitted; Water pressure; All air vented; Pump / harness / connections; Water leaks / blockages; Safety thermostats / low voltage wiring harness / connections; Blockage in flue system.
What causes this fault?
This fault occurs when the boiler's internal temperature exceeds safe limits or when exhaust gases cannot escape freely through the chimney pipe. The most common reasons include a failing pump that isn't moving water fast enough, a build-up of debris in the heat exchanger, or a physical obstruction in the flue system.
This issue often surfaces during the first cold snap of autumn when the heating is turned back on after months of inactivity, causing seized pumps or trapped air to trigger an overheat.
DIY fix possible
Check the pressure gauge is between 1 and 1.5 barIdentify and open any closed radiator valvesConfirm there are no external obstructions blocking the flue terminal outside
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 Slow flash (normally on, flashes off) — no call-out fees, no surprises.
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Worcester Bosch Greenstar 24i Junior 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.
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