Low
18 on Viessmann Vitodens 300-W

Your boiler has lost connection to its outdoor sensor, causing it to think it is freezing outside and potentially making your radiators warmer than necessary.

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.

What the manual says

Manufacturer manual

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

From this model's manual
Verified against the Viessmann Vitodens 200 W installation & service manual, page 92.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

DIY fix possible

Locate the outdoor sensor on your external wallCheck for any visible damage or loose wires leading to the sensor unitEnsure the sensor is not covered by debris or snowReset the boiler to see if the fault clears

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