BoilerFaultCodesYour boiler has shut down because it is heating up much faster than it can circulate water, indicating a blockage or a pump issue.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Lock-out: maximum temperature rise is exceeded, dT between supply and return >= 22°C, rate of rise > 2,7°C/sec, or start with no flow/insufficient water. Also activated if shut down input (terminals 5 and 6) is opened.
What causes this fault?
This fault occurs when heat builds up inside the boiler faster than the system can carry it away to your radiators. The most common reasons are a failed internal pump that isn't pushing water through the pipes or a physical blockage, such as sludge or debris, restricting the flow. Essentially, the boiler is protecting itself from overheating because the water is essentially standing still while being heated.
This issue often comes to light in autumn when the heating is turned back on after a long summer break, as pump components can seize during periods of inactivity.
DIY fix possible
Check that all radiator valves are fully openEnsure the boiler pressure is between 1.0 and 1.5 barBleed all radiators to remove trapped air from the system
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A boiler cover plan would cover faults like b9 — no call-out fees, no surprises.
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