BoilerFaultCodesViessmann 34:1 Fault Code
Also written as 341 — same fault.
The Viessmann 34:1 fault code means: your boiler is confused by an external signal telling it to run the heating pump, which may be caused by a faulty control module or a wiring problem.. It appears on 5 Viessmann models in our database.
Technical description: Signal "External demand" on circulation pumps: See table in manual (Address 34:1)
What causes the Viessmann 34:1 fault?
This fault happens because the boiler is receiving a request for heat from an external pump or accessory, but its internal settings are not configured to recognize that specific signal. It is essentially a communication mismatch where the boiler’s software is told to expect one type of instruction but receives another. This usually occurs following a new installation or a control system upgrade where the technician forgot to update the digital 'address' in the boiler's menu.
DIY fix possible
Locate the reset button on the front control panel.Press and hold the reset button for 2 to 3 seconds.Wait for the boiler to restart; if the code returns, contact a professional to check the system configuration.
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Gas Safe Register →Viessmann models that show the 34:1 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 34:1 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