BoilerFaultCodesYour boiler has shut down because its internal temperature has reached a dangerously high level, triggering a safety switch 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: Limit thermostat fault
What causes this fault?
This fault occurs when the water inside the boiler gets too hot, causing a safety sensor to trip and shut the system down to prevent internal damage. The most common reasons are a lack of water flow due to a failing pump, a blockage in the heat exchanger, or a buildup of air within the pipes.
This issue is more common during the first few cold days of autumn when a pump that has been idle all summer suddenly seizes up upon being restarted.
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.
Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like AL20 — 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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