BoilerFaultCodesYour boiler has shut down because the internal water has become too hot or isn't circulating through the system quickly enough.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Flow temperature too high, or temperature differential between primary and return too high
What causes this fault?
This fault occurs when the water inside the boiler heats up much faster than it can be moved away into your radiators. The most common cause is a blockage in the system or a failing pump that is no longer strong enough to keep the water circulating. Because the heat cannot 'escape' the boiler, the safety sensors shut everything down to prevent damage from overheating.
This issue often surfaces during the first cold spell of autumn when the heating is turned back on after months of inactivity, which can cause the internal pump to seize.
DIY fix possible
Ensure all radiator valves (TRVs) are turned to the maximum settingCheck that the central heating valves underneath the boiler are fully openCheck the system pressure gauge and top up to 1.5 bar if it is lowTurn the boiler off and back on to attempt a reset
Find a Gas Safe engineer
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A boiler cover plan would cover faults like AL79 — 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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