How much does it cost to fix Vaillant error code D.17?

Typical repair cost for the Vaillant D.17 fault is £90-250 including labour.

D.17 Low

Vaillant D.17 Fault Code

Q: What does the Vaillant D.17 fault code mean?

Your boiler is switching from controlling the water temperature based on the outgoing flow to the returning water because it has detected a more stable reading that way.

Also written as D17 — same fault.

The Vaillant D.17 fault code means: your boiler is switching from controlling the water temperature based on the outgoing flow to the returning water because it has detected a more stable reading that way.. It appears on 25 Vaillant models in our database.

Technical description: Heating glow return regulation

What causes the Vaillant D.17 fault?

This occurs when the boiler detects that the temperature sensor on the return pipe is providing a more reliable reading than the one on the flow pipe. It physically happens because the control board senses a conflict or inconsistency in how heat is being measured as water leaves the unit, prompting it to switch its monitoring method to ensure safe operation. The most common reasons are a drifting calibration in a sensor or a slight restriction in water flow through the heat exchanger.

Typical repair cost £90-250 (including labour)

Engineer required

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

Find a Gas Safe engineer

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

Gas Safe Register →

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 27 April 2026