d.77 Medium

Vaillant d.77 Fault Code

Also written as D77 — same fault.

The Vaillant d.77 fault code means: your boiler is struggling to bring your hot water tank up to the correct temperature, likely due to a sensor issue or a problem with the pump transferring heat to the cylinder.. It appears on 10 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: Max. cylinder reheating
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.77 fault code

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

Source

Vaillant d.77 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