High Lockout
A8 on Worcester Bosch Greenstar 25 HE Combi

Your boiler’s internal computer and main controls have lost their connection, preventing the system from starting up.

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

Break in communication to the TR2 room thermostat

What causes this fault?

This fault occurs when the 'brain' of your boiler can no longer talk to the control panel or the internal sensors. It is usually caused by moisture affecting the electrical circuits, a loose connection in the internal wiring, or a critical failure of the main printed circuit board itself.

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

E9View →A1View →C6View →

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