BoilerFaultCodesYour boiler is struggling to operate because the electrical power reaching it is unstable, insufficient, or the control module has failed to recognize the incoming voltage.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Low mains voltage – the voltage may be interrupted, inconsistent or heat control module (HCM) invalid
What causes this fault?
This fault occurs when the boiler's internal electronics detect that the incoming electrical supply is too weak or unstable to safely power the components. It is often caused by a faulty Heat Control Module (HCM) failing to read the voltage correctly, though it can also be triggered by a genuine dip in the property's mains power supply or a failing main circuit board.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
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