BoilerFaultCodesViessmann 9b:1 to 9b:127 Fault Code
Also written as 9B1TO9B127 — same fault.
The Viessmann 9b:1 to 9b:127 fault code means: your boiler has detected an internal configuration error where the target temperature has been set to an invalid value or a internal circuit board fault is occurring.. It appears on 5 Viessmann models in our database.
Technical description: Set temperature adjustable from 1 to 127°C
What causes the Viessmann 9b:1 to 9b:127 fault?
This fault occurs when the boiler's internal computer receives conflicting or impossible data regarding the temperatures it is supposed to maintain. It is usually caused by a communication failure between the main control board and the electronic coding plug that stores the boiler's specific settings. In some cases, damaged internal wiring or a failing circuit board can also lead to these incorrect readings.
DIY fix possible
Turn the boiler power off at the fused spur switch for 30 seconds and then back on.Attempt to reset the boiler using the control panel interface.Navigate to the heating settings and ensure the target temperature is set within a normal range (e.g., 40-75°C).
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Gas Safe Register →Viessmann models that show the 9b:1 to 9b:127 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 9b:1 to 9b:127 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