d.88 Medium

Vaillant d.88 Fault Code

Also written as D88 — same fault.

The Vaillant d.88 fault code means: your boiler is struggling to detect the flow of water needed to start heating your hot water tap, likely due to a sensor issue or a drop in water pressure.. It appears on 8 Vaillant models in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: Flow rate limit value for ignition in domestic hot water mode
Typical repair cost £120-250 (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.88 fault code

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

Source

Vaillant d.88 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