BoilerFaultCodesYour hot water tank has overheated and a safety switch has shut the system down to prevent damage.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: The cylinder temperature is so high that the cylinder High Limit Thermal Cut-out has tripped.
What causes this fault?
This fault occurs when the water inside your hot water storage cylinder reaches an unsafely high temperature, triggering a safety switch to kill the power to the boiler. The most common reason is a mechanical failure in the motorised zone valve, which gets stuck in the 'open' position and continues to heat the tank even when the thermostat tells it to stop.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
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A boiler cover plan would cover faults like Fault 12 — no call-out fees, no surprises.
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Could it be time for a new boiler?
If repair costs are adding up, a new energy-efficient boiler could be the smarter long-term choice.
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