High Lockout
A2 on Viessmann Vitodens 050-W

Your boiler has sensed that the internal pump is malfunctioning or water is not circulating correctly, which has caused the system to stop as a safety precaution.

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

N/A

What causes this fault?

This fault occurs when the boiler's internal sensors detect that water is not moving through the heat exchanger as quickly or efficiently as it should. The most common reason is a mechanical failure of the circulation pump or a buildup of debris that has physically jammed the pump's internal components. In some cases, it may also be triggered by a communication error where the main control board is unable to tell the pump to spin.

This fault is more frequent in autumn when the heating is turned back on after a long summer break, as pumps can often seize up during months of inactivity.

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

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