BoilerFaultCodesViessmann 48 Fault Code
The Viessmann 48 fault code means: your boiler is having trouble controlling the temperature for one of your heating zones, likely due to a faulty sensor reading.. It appears on 7 Viessmann models in our database.
Technical description: Mixer closes flow temperature sensor, heating circuit 2 (with mixer)
What causes the Viessmann 48 fault?
This fault occurs when the communication link between your boiler and the hot water tank is broken, meaning the boiler no longer knows how hot your water is. The most common reason is that the internal wiring of the temperature sensor has become brittle or snapped, or the sensor itself has suffered an electrical failure. Because the boiler cannot safely monitor the temperature to prevent overheating, it stops providing hot water as a safety precaution.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Viessmann models that show the 48 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 48 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 manual (PDF)
- Viessmann Vitodens 100-W WB1A manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026