BoilerFaultCodesYour boiler is struggling to switch between providing heating and hot water because of an internal valve check failure.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Three way valve test
What causes this fault?
This fault occurs when the boiler's internal computer tries to move the diverter valve to switch between heating and hot water, but the movement isn't detected correctly. The most common reason is that the valve has become stuck due to a buildup of debris in the system or the small electric motor that moves the valve has failed.
This fault is frequently seen at the start of autumn when the heating is turned back on after being idle all summer, which can cause internal moving parts 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 T4 — no call-out fees, no surprises.
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