BoilerFaultCodesYour boiler has detected that water is moving too slowly or not at all through the system, causing a heat imbalance that has triggered a safety shutdown.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Maximum difference between T1 and T2 exceeded
What causes this fault?
This fault occurs when the boiler's internal sensors detect that the outgoing water is significantly hotter than the returning water, suggesting a lack of circulation. The most common reason is a failed or stuck pump, though it can also be triggered by air trapped in the system or a blockage in the pipework. In some cases, a build-up of sludge or debris within the heat exchanger prevents the water from moving through the boiler fast enough to carry the heat away.
This fault often appears during the first few cold days of autumn when a pump that has been idle all summer fails to spin back up.
DIY fix possible
Check that your central heating valves and all radiator valves are fully openEnsure there is no air trapped in the system by bleeding your radiatorsCheck if the system pressure gauge is between 1 and 1.5 bar
Find a Gas Safe engineer
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A boiler cover plan would cover faults like 430 — no call-out fees, no surprises.
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