d.16 High

Vaillant d.16 Fault Code

Also written as D16 — same fault.

The Vaillant d.16 fault code means: your boiler is struggling to push heated water around your radiators because an external pump is not responding correctly.. It appears on 18 Vaillant models in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description: External CH pump

What causes the Vaillant d.16 fault?

This fault occurs when the boiler sends a signal to an external pump to start moving water through your radiators, but it detects that no power is being used or the pump is failing to spin. The most common reasons are a seized pump motor after being left off for a long time, or a loose wiring connection between the boiler's control board and the pump itself.

This issue is frequently seen during the first cold spell of autumn when a pump that has sat idle all summer becomes stuck due to internal debris or limescale.

Typical repair cost £180-350 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Vaillant models that show the d.16 fault code

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

Source

Vaillant d.16 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