Low
S.23 on Vaillant ecoTEC Pro 24

Your boiler is simply going through its normal process of lighting the burner to provide you with hot water.

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

DHW mode: Ignition. The ignition for domestic hot water mode is activated.

Shared across the range
This code is shared by the boilers in this range, which use the same control board. It may not be printed in this exact variant’s manual.
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 is a normal status code rather than a fault, occurring every time you turn on a hot tap. It indicates that the boiler's internal computer has successfully detected a request for hot water and is now physically sparking the burner to heat it. Once the flame is established and the water reaches the set temperature, this code will typically transition to another status.

DIY fix possible

This fault may be fixable without an engineer. Check your manual first.

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Avoid the next call-out bill

A cover plan would handle faults like S.23: no call-out fees, no surprise bills.

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