D.17 Low

Vaillant D.17 Fault Code

Also written as D17 — same fault.

The Vaillant D.17 fault code means: your boiler has switched its control method to the return pipe because it is detecting a potential issue with the main flow sensor.. It appears on 11 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 flow return regulation

What causes the Vaillant D.17 fault?

This fault occurs when the boiler detects a logical error or an inconsistent reading from the flow temperature sensor. To keep your heating running safely, the boiler automatically switches to monitoring the return pipe sensor while it ignores the potentially faulty flow data. The most common reason is a failing or 'drifting' sensor that is sending incorrect electrical signals back to the main control board.

Typical repair cost £90-180 (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.17 fault code

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

Source

Vaillant D.17 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 31 August 2026