D.166 High

Vaillant D.166 Fault Code

Also written as D166 — same fault.

The Vaillant D.166 fault code means: your boiler's internal computer is struggling to read data from its sensors, which prevents it from safely controlling your heating and hot water.. It appears on 3 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: ADCfaultindex
Typical repair cost £250-450 (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.166 fault code

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

Source

Vaillant D.166 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