BoilerFaultCodesViessmann 28 Fault Code
The Viessmann 28 fault code means: your boiler has lost communication with a sensor that monitors the temperature of the water flowing into your heating system, usually because of a loose or damaged wire.. It appears on 9 Viessmann models in our database.
Technical description: Controls without flow temperature sensor (low loss header) - System flow temperature sensor lead break
What causes the Viessmann 28 fault?
This fault occurs when the boiler's control board can no longer detect a signal from the temperature sensor located on the main pipework. The most common reason is a physical break in the internal wiring or a sensor that has failed due to age, preventing the boiler from knowing how hot the water is as it leaves the unit. Because the system cannot safely monitor the heat, it shuts down to prevent damage.
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 28 fault code
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Source
Viessmann 28 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