BoilerFaultCodesYour boiler has shut down because it has detected internal temperatures that are too high, triggered by a safety sensor to prevent overheating.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: High limit stat
What causes this fault?
This fault occurs when the boiler's internal water temperature exceeds safe operating limits, causing a safety sensor to trip and shut the system down. The most common reasons are a pump failure that stops water from moving or a buildup of limescale and sludge that restricts flow through the heat exchanger. Essentially, heat is being generated faster than the water can carry it away to your radiators.
This fault is more common in autumn when heating systems are turned on for the first time after summer, as circulation pumps can seize during long periods of inactivity.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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A boiler cover plan would cover faults like 20-21 — no call-out fees, no surprises.
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