Medium Lockout
9b:1 to 9b:127 on Viessmann Vitodens 300-W

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.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description

Set temperature adjustable from 1 to 127°C

What causes this 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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Related fault codes

9bView →b0View →b1View →F4View →

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