BoilerFaultCodesYour boiler has stopped working because it has failed to light or has detected a dangerous build-up of heat.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Ignition lockout. Overheat lockout. Internal error. (Common causes: No gas or low gas pressure, Weak spark, Gas valve leaking or failed, Low water pressure or boiler dry, Pump fail, Primary sensor position, Flame sensor fail, Overheat stat tripped, Blocked burner, Blocked heat exchanger, Faulty harness, Board fault).
What causes this fault?
This fault occurs when the boiler tries to light but fails to detect a flame, or if the internal temperature becomes unsafely high. It is often caused by a lack of fuel reaching the burner, a worn-out ignition spark, or a pump failure that prevents water from moving, leading to immediate overheating.
This issue often surfaces during the first cold snap of autumn when the pump or gas valve seizes up after being inactive over the summer.
DIY fix possible
Check that your gas supply is turned on (e.g., check other gas appliances)Check the pressure gauge is between 1.0 and 1.5 barPress and hold the reset button for five seconds
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
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A boiler cover plan would cover faults like Flash once per second — no call-out fees, no surprises.
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Worcester Bosch 24si II 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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