D.0 Low

Vaillant D.0 Fault Code

Also written as D0 — same fault.

The Vaillant D.0 fault code means: your boiler is currently restricted to a lower power setting for heating, meaning your radiators might take longer to warm up or may not get as hot as they should.. It appears on 14 Vaillant models in our database.

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: Heating part load error

What causes the Vaillant D.0 fault?

This occurs when the boiler's software has been manually adjusted to limit how much gas it uses for the heating system. It is usually set by an engineer during installation to match the size of your home, or it may have been limited accidentally during a service. Because of this limit, the boiler cannot use its full power to heat your radiators even if the house feels cold.

This issue is most noticeable during the first cold snap of winter when the boiler's restricted power output struggles to keep up with the dropping outside temperatures.

Typical repair cost £0-80 (including labour)

DIY fix possible

Check if any radiator valves are turned off or restrictedMonitor if the home reaches the desired temperature over a longer periodNo immediate action is required if your home remains comfortable

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Vaillant models that show the D.0 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Vaillant D.0 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026