BoilerFaultCodesYour boiler has shut down because its internal safety sensors detected that it was beginning to overheat.
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 water inside the boiler reaches an unsafe temperature, triggering a safety switch to prevent damage. The most common reasons are a failed pump that isn't circulating water properly, or a buildup of sludge and limescale that blocks the heat exchanger. It can also be caused by a simple sensor failure where the boiler incorrectly thinks it is overheating.
This issue often surfaces when the heating is first turned on after summer, as pump components can seize up during long periods of inactivity.
DIY fix possible
Ensure all radiator valves are fully open to allow water to circulateCheck that the boiler pressure is between 1.0 and 1.5 bar on the gaugeAllow the boiler to cool down, then turn the selector switch to 'R' for five seconds to reset
Find a Gas Safe engineer
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A boiler cover plan would cover faults like 21 — 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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