BoilerFaultCodesYour boiler has switched itself off because it is getting too hot inside, likely due to a circulation problem or a sensor fault.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Safety Thermostat Activated (Boiler or Flue)
What causes this fault?
This fault occurs when the boiler's internal temperature exceeds a safe limit, triggering a safety switch to prevent damage. It is usually caused by water failing to move around the system quickly enough to carry heat away, often due to a pump failure or a blockage. In some cases, the sensor itself may have failed and is reporting a high temperature even if the boiler is cool.
This fault is more common when owners first turn their heating back on in autumn, as circulation pumps can seize up after being inactive over the summer.
DIY fix possible
Check that all radiator valves are fully openEnsure your system pressure is between 1.0 and 1.5 bar on the gaugeTurn the selector switch to 'Reset' for five seconds and then back to the original position
Find a Gas Safe engineer
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A boiler cover plan would cover faults like 25 — no call-out fees, no surprises.
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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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