BoilerFaultCodesYour boiler has likely overheated or a safety sensor is failing, causing the system to lock out 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: Flow sensor fault / high limit intervention
What causes this fault?
This fault occurs when the boiler's internal temperature gets too high or the sensor tasked with monitoring that heat fails. The most common reasons are a lack of water flow through the system due to a pump failure, a blockage in the heat exchanger, or a faulty safety sensor providing an incorrect reading.
This issue often appears when the heating is first turned on after summer, as internal components like the pump can seize during long periods of inactivity.
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 AL79 — no call-out fees, no surprises.
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