BoilerFaultCodesViessmann 94 Fault Code
The Viessmann 94 fault code means: your boiler has lost communication with the temperature sensor in your hot water tank because of an electrical fault, meaning it cannot accurately monitor your solar heated water.. It appears on 5 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 electrical circuit for the solar hot water sensor is completed prematurely, usually due to moisture ingress or damaged insulation. The most common reason is that the sensor probe has failed internally or the wires connecting it to the boiler have become pinched or corroded, preventing the system from reading the water temperature.
This fault is more common during the summer months when solar thermal systems are under peak thermal load and high temperatures can accelerate the degradation of aging sensor cables.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Avoid the next call-out bill
A cover plan would handle faults like 94: no call-out fees, no surprise bills.
Affiliate links. We may earn a commission at no extra cost to you.
Viessmann models that show the 94 fault code
Click a model for the full fault code list and model-specific repair notes.
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.
- Viessmann Vitodens 050-W manual (PDF)
- Viessmann Vitodens 100-W WB1A manual (PDF)
- Viessmann Vitodens 222-F manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026