BoilerFaultCodesViessmann 73:7 Fault Code
Also written as 737 — same fault.
The Viessmann 73:7 fault code means: your boiler has detected a communication error with its internal control system or a connected smart module, which may prevent it from managing your heating correctly.. It appears on 5 Viessmann models in our database.
Technical description: Constantly ON
What causes the Viessmann 73:7 fault?
This fault occurs when the boiler's internal communication hardware suffers a permanent electronic failure, meaning it can no longer send or receive data from the control system. The most common reason is a short circuit or component degradation within the LON module or the main control board itself, which prevents the boiler from identifying its operating instructions. Because the system can no longer 'talk' to its sensors and switches, it shuts down for safety.
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 73:7 fault code
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Source
Viessmann 73:7 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