34:2 Medium

Viessmann 34:2 Fault Code

Also written as 342 — same fault.

The Viessmann 34:2 fault code means: 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.. It appears on 5 Viessmann models in our database.

Book a service

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 the Viessmann 34:2 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.

Typical repair cost £120-350 (including labour)

Engineer required

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

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Viessmann models that show the 34:2 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Viessmann 34:2 fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026