d.18 Low

Vaillant d.18 Fault Code

Also written as D18 — same fault.

The Vaillant d.18 fault code means: your boiler is simply showing its current pump setting status rather than a fault; it indicates whether the pump is running constantly or only when needed.. It appears on 19 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: Pump mode (Eco/Comfort)

What causes the Vaillant d.18 fault?

This is not a mechanical failure but rather a status indicator regarding how your internal circulation pump is programmed to behave. It appears when the boiler is set to either run the pump continuously for constant heat circulation or only activate it during a specific heating demand. There is no physical component at fault; the boiler is simply communicating its current operating logic.

DIY fix possible

Press the 'i' or 'Back' button to exit the information menuNo further action is required as this is a status code, not an errorTo change the mode, enter the installer level settings (only recommended if you have your manual)

Find a Gas Safe engineer

Search the official register for a qualified engineer in your area.

Gas Safe Register →
Boiler cover options

Avoid the next call-out bill

A cover plan would handle faults like d.18: no call-out fees, no surprise bills.

Approved partner
Domestic Appliance Guard
Unlimited call-outs, 24/7 helpline, no price-hike promise
Get a quote →

Affiliate links. We may earn a commission at no extra cost to you.

Vaillant models that show the d.18 fault code

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

Source

Vaillant d.18 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 7 July 2026