Low
S.34 on Vaillant ecoTEC Plus 620

Your boiler has sensed the temperature is very cold and is automatically running to prevent the internal pipes from freezing.

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

Anti frost mode active

From this model's manual
Verified against the Vaillant Ecotec Plus 615 installation & service manual, page 39.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

What causes this fault?

This status occurs when the internal sensors detect that the water temperature inside the boiler has dropped below a safe threshold, usually around 5°C. To protect itself from damage, the boiler automatically fires up to circulate warm water and prevent the heat exchanger from freezing solid. It is a safety feature rather than a mechanical failure.

This code is almost exclusively seen during the winter months when outdoor temperatures drop near or below freezing.

DIY fix possible

Wait for the boiler to finish its cycle; the status will clear once the internal temperature rises.Ensure your thermostat is turned up to your desired temperature.Check that any external pipes, such as the condensate pipe, are not frozen solid.

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