BoilerFaultCodesYour boiler is providing central heating but is failing to detect when you turn on a hot tap, meaning you have no hot water.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Flow sensor failure. DHW sensor (LED 4 flash/second). Board fault.
What causes this fault?
This fault occurs when the boiler's internal sensors fail to detect water moving through the system when you open a tap. The most common reason is that the plastic turbine inside the flow sensor has become stuck with limescale or debris, preventing it from sending a signal to the boiler to fire up for hot water. In some cases, the temperature sensor has simply worn out and is providing an incorrect reading that tricks the system into staying off.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
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A boiler cover plan would cover faults like No DHW function (CH OK) — no call-out fees, no surprises.
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Worcester Bosch 24si II is discontinued — time for a new one?
Parts for discontinued models can be hard to find and expensive. A new A-rated boiler could save you money on both repairs and energy bills.
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