High Lockout
A8 on Worcester Bosch CDi Classic

Your boiler has lost its connection to the external heating controls, meaning it cannot receive instructions on when to turn on or off.

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 communication to FX sensor controls electrical connections need checking.

What causes this fault?

This fault occurs when the boiler's internal computer loses its digital handshake with your room thermostat or programmer. The most common reason is a hardware failure in the wireless receiver plugged into the boiler front, or degraded electrical wiring that physically links the controls to the 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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Related fault codes

A7View →B1View →

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