BoilerFaultCodesYour boiler has shut down because it has detected its internal parts getting too hot, likely due to a lack of water circulation or a blockage.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Heat exchanger overheat
What causes this fault?
This fault occurs when the water inside the boiler reaches an unsafe temperature because it isn't moving away from the heat source quickly enough. The most common reason is a failing internal pump or a buildup of sludge and debris that is physically blocking the flow of water through the system. In some cases, it can also be caused by an air lock or a faulty sensor incorrectly reading the temperature.
This issue often surfaces when the heating is first turned on in autumn, as pump components can seize up after being inactive over the summer months.
DIY fix possible
Ensure your radiator valves are all open to allow water to flow.Check the boiler pressure gauge and top it up to 1.5 bar if it is low.Press and hold the Reset button for five seconds to see if the fault clears.
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), Lockout reset: Flashing (reset required) (Alt 1) — no call-out fees, no surprises.
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Worcester Bosch Greenstar 25Si 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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