BoilerFaultCodesViessmann 34:0 Fault Code
Also written as 340 — same fault.
The Viessmann 34:0 fault code means: your boiler is receiving a constant request for heat from an external control or thermostat, causing all the pumps to run continuously.. It appears on 6 Viessmann models in our database.
Technical description: Influence of 'External demand' signal on circulation pumps: All pumps are controlled
What causes the Viessmann 34:0 fault?
This fault happens because the boiler is receiving a permanent electrical signal telling it to keep heating, even when your heating is turned off. It is usually caused by a physical fault in an external part, such as a stuck motorized valve or a faulty room thermostat that is failing to stop the demand signal. Because the boiler thinks it is still being asked for heat, it keeps the internal pumps running constantly to circulate water.
DIY fix possible
Check all room thermostats are not set excessively highCheck if the programmers or timers are stuck on an 'Always On' settingTurn the boiler power off and back on at the fused spur to see if the signal resets
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Gas Safe Register →Viessmann models that show the 34:0 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 34:0 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