Low
d.25 on Vaillant ecoTEC sustain

Your boiler is simply indicating that the 'warm start' or cylinder heating feature requested by your thermostat is currently turned off.

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

Status of the requirement to reheat the cylinder or for the hot water warm start from the eBUS thermostat: off=Function deactivated, on=Function activated

From this model's manual
Verified against the Vaillant Ecotec Sustain installation & service manual, page 47.
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

Check your thermostat or boiler display settings to see if the 'warm start' or 'hot water comfort' function is disabledToggle the setting to 'on' if you prefer your hot water to be kept warm for faster deliveryNo action is required if you are happy with how your hot water is currently functioning

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