Medium Lockout
80:1 on Viessmann Vitodens 300-W

Your boiler has detected a persistent communication issue with its external sensors or internal control system that has lasted more than five seconds.

Book a service

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

With 5 seconds' time delay for fault message; message will be issued, if a fault persists for at least 5 seconds

What causes this fault?

This fault occurs when the boiler's central brain loses contact with the sensors that monitor water temperature. It is usually caused by a loose connection in the internal wiring or a physical failure within the temperature sensor itself, meaning the boiler can no longer safely verify how hot it is. In some cases, the control board may be unable to process the data it receives, leading to a total communication breakdown.

DIY fix possible

Navigate to the boiler control panelPerform a system reset by pressing the RESET buttonCheck for any loose external wiring if visible and safe to touch

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