BoilerFaultCodesViessmann 94 Fault Code
The Viessmann 94 fault code means: your boiler has detected a problem with the sensor that measures the water temperature in your solar cylinder, which may affect how your hot water is heated.. It appears on 7 Viessmann models in our database.
Technical description: Control mode - Solar: Cylinder temperature sensor shorted out
What causes the Viessmann 94 fault?
This fault occurs when the sensor responsible for monitoring the temperature of your solar water cylinder develops an electrical fault, causing it to send an incorrect signal to the boiler's control board. The most common reason is that the sensor's internal wiring has degraded over time or moisture has caused a short circuit, preventing the system from knowing if the water is hot enough. In some cases, it can also be triggered by a loose or damaged connection in the wiring harness that links the sensor to the main unit.
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 94 fault code
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Source
Viessmann 94 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