High Lockout
9U233 on Worcester Bosch CDi Classic

Your boiler's main internal computer or memory chip has developed a communication fault or has failed, meaning the unit cannot safely control the heating process.

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

Control box or heat control module (HCM) is defective or loose, the HCM needs to be checked

What causes this fault?

This fault occurs when the boiler's central brain loses contact with its memory chip, which stores the specific settings required for your model to operate. It is usually caused by a loose connection where the chip plugs into the circuit board, or the internal electronic components have simply worn out and failed over time.

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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