Low Warning
18 on Viessmann Vitodens 333

Your boiler is confused about the weather outside because the external sensor has lost its connection, meaning the heating might not adjust correctly to the outdoor temperature.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description

Controls according to 0°C outside temperature - Outside temperature sensor lead break

What causes this fault?

This fault occurs when the electrical link between the boiler's control board and the sensor located on your outside wall is interrupted. The most common reasons are a loose connection at the terminal, a break in the thin communication wire, or internal corrosion within the sensor housing caused by moisture over time.

This fault often becomes apparent during autumn or winter when the heating is first turned on and the boiler tries to calculate its output based on the drop in outdoor temperature.

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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