High Lockout
C5 on Viessmann Vitodens 050-W

Your boiler is having trouble communicating with its internal pump, which means it cannot properly move heated water around your system.

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

Control mode, max. pump speed: Communication fault - variable speed internal pump

What causes this fault?

This fault occurs when the boiler's main control board loses its digital connection with the internal pump, preventing the system from regulating water flow. The most common reason is a failure in the pump's electronic circuitry or a loose connection in the wiring harness that carries data signals. Without this communication, the boiler cannot safely manage heat distribution and will stop operating to prevent overheating.

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

C6View →F9View →

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