BoilerFaultCodesViessmann d1 Fault Code
Also written as D1 — same fault.
The Viessmann d1 fault code means: your boiler has stopped working because it cannot detect any water moving through the system to keep it from overheating.. It appears on 5 Viessmann models in our database.
Technical description: Burner fault: Flow switch has not responded
What causes the Viessmann d1 fault?
This fault occurs when the boiler's internal sensors fail to detect water circulating through the heat exchanger after the pump has started. The most common cause is a seized pump or a blockage in the pipework that prevents the flow switch from physically moving into the 'on' position. If the boiler cannot confirm that water is moving to carry heat away, it shuts down immediately to prevent the unit from melting or exploding.
This issue is more frequent in autumn when the heating is turned on for the first time after summer, as internal pump components can seize up during long periods of inactivity.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Viessmann models that show the d1 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann d1 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