High Lockout
Red light (flashing) on Glow-worm Swift Flow 100

Your boiler has encountered an internal error and needs a simple restart to try and clear the fault.

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

General fault; requires momentary interruption of the electrical supply to reset.

What causes this fault?

This fault occurs when the boiler's internal electronics detect a communication error or a safety interruption that prevents the system from starting. The most common reason is a temporary glitch in the control board or a sensor that has tripped due to minor overheating or a failed ignition attempt. It acts as a safety barrier to ensure the boiler does not operate until a qualified person or a manual reset has cleared the issue.

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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