BoilerFaultCodesYour boiler has detected that water is not circulating correctly through the system, preventing it from heating up safely.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Differential water flow switch fault (still open).
What causes this fault?
This fault occurs when the boiler's internal sensors detect that water isn't moving through the unit quickly enough to carry heat away safely. The most common reasons are a seized circulation pump, a blockage in the pipework, or a failure in the switch that monitors water flow. Because the heat cannot exit the boiler, the system shuts down to prevent the internal components from overheating.
This issue frequently appears during the first cold snap of autumn when a pump that has been idle all summer fails to restart or is hindered by settled debris.
DIY fix possible
Check that your boiler pressure is between 1.0 and 1.5 bar on the gaugeEnsure all radiator valves and isolation valves under the boiler are fully openBleed your radiators to remove any trapped air pocketsReset the boiler by turning the selector switch to 'R' for five seconds
Find a Gas Safe engineer
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A boiler cover plan would cover faults like E12 — no call-out fees, no surprises.
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Could it be time for a new boiler?
If repair costs are adding up, a new energy-efficient boiler could be the smarter long-term choice.
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