BoilerFaultCodesYour boiler has switched itself off because it has reached an unsafe temperature and needs to cool down and be reset.
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
What causes this fault?
This fault occurs when the water inside the boiler gets too hot, triggering a safety switch to prevent damage. The most common reasons are a pump failure preventing water from circulating or a buildup of sludge and debris blocking the internal pipework. In some cases, the sensor itself may have failed and is reporting an incorrect temperature.
This fault is more frequent in autumn when heating systems are turned back on after summer, as pump components can seize during long periods of inactivity.
DIY fix possible
Wait for the boiler to cool down for at least 20 minutesTurn the control knob to the 'R' (Reset) position and hold for a few secondsReturn the knob to the original 'on' position
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A boiler cover plan would cover faults like 21 — no call-out fees, no surprises.
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