BoilerFaultCodesYour boiler is unable to move water around the system, which is likely caused by a stuck internal pump or a closed valve blocking the flow.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Pump seized / Flow/return service valve closed
What causes this fault?
This fault occurs when the boiler detects that water is not circulating through the heat exchanger, triggering a safety shutdown to prevent overheating. The most common cause is the internal pump becoming seized or stuck, often due to a build-up of debris or the pump capacitor failing. It can also happen if the service valves underneath the boiler have been accidentally closed, physically blocking the path of the water.
This issue is particularly common in autumn when heating systems are turned back on after being idle all summer, as pumps often seize during long periods of inactivity.
DIY fix possible
Check that the service valves underneath the boiler (usually handle-style taps) are fully open and parallel to the pipework.Ensure all radiator valves are turned to their highest setting to allow water to flow.Try resetting the boiler to see if the pump clears its own blockage.
Find a Gas Safe engineer
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A boiler cover plan would cover faults like b/E 18 — no call-out fees, no surprises.
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