BoilerFaultCodesYour boiler is not starting because it cannot detect water moving through the system to prevent overheating.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: The flow switch terminals are left disconnected and the boiler will not operate.
What causes this fault?
This fault occurs when the boiler's internal safety sensor fails to detect water circulating through the heat exchanger. The most common reasons are a physically stuck flow switch, a failed circulation pump that isn't moving the water, or a loose electrical connection in the wiring harness. Without this movement, the boiler shuts down immediately to prevent the internal components from overheating and sustaining damage.
This issue often surfaces at the start of autumn when heating systems are turned back on after sitting idle all summer, which can cause the pump or switch to seize.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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A boiler cover plan would cover faults like No flow — no call-out fees, no surprises.
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