Medium Warning
C7 on Viessmann Vitodens 333

Your boiler is having trouble communicating with the internal pump that moves water around your radiators, which may lead to your heating not working effectively.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description

Communication fault - variable speed heating circuit pump, heating circuit A1. Control mode, max. pump speed

What causes this fault?

This fault occurs when the boiler's main computer loses its digital connection with the high-efficiency circulation pump. The most common reason is a technical glitch in the pump's internal electronics or a loose connection in the wiring that carries speed signals. Without this communication, the boiler cannot safely coordinate how fast water should move through your radiators.

This issue often surfaces at the start of autumn when the pump, having been idle over the summer, is suddenly required to work at varying speeds again.

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Related fault codes

C6View →F9View →

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