Medium Warning
34:2 on Viessmann Vitodens 300-W

Your boiler has detected an unusual communication signal from an external control or pump, meaning the system is confused about when it should be running.

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

Signal "External demand" on circulation pumps: See table in manual (Address 34:2)

What causes this fault?

This fault occurs when the boiler receives conflicting or illogical instructions from an external control unit regarding how fast the internal pumps should be running. The most common reason is a communication error between the main circuit board and an add-on module or a smart thermostat that has been incorrectly configured. It essentially means the boiler’s brain is confused by the 'speed request' it is getting from your heating controls.

Engineer required

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

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Related fault codes

34:1View →

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