High Lockout
A8 on Worcester Bosch ZWB 7-25

Your boiler's internal control system has lost its connection, meaning it cannot properly talk to its own brain to coordinate heating or hot water.

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

Communication is interrupted.

What causes this fault?

This fault occurs when the boiler's internal computer can no longer 'talk' to the sensors or the thermostat that tells it when to turn on. The most common reason is a loose or damaged wiring connection inside the unit, or a internal failure on the main circuit board itself. This break in communication means the boiler enters a safety mode because it cannot guarantee it is receiving the correct operating instructions.

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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Related fault codes

A7View →

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