High Lockout
A8362 on Worcester Bosch CDi Classic

Your 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.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 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 is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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