F.77 High

Vaillant F.77 Fault Code

Also written as F77 — same fault.

The Vaillant F.77 fault code means: your boiler has detected an issue with the drainage system that removes leftover liquid from the exhaust pipe or a fault with the safety flap inside the flue.. It appears on 35 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: Flue non return flap condensate pump error

What causes the Vaillant F.77 fault?

This fault occurs when the boiler's internal safety sensors detect that water is backing up in the flue system or that the mechanical flap designed to stop exhaust fumes from returning is stuck. The most common reason is a failed condensate pump that can no longer push waste liquid away, or debris interfering with the flue flap's movement. Because this affects how exhaust gases and liquids leave the unit, the boiler shuts down as a safety precaution.

This fault is more frequent during particularly cold winters, as frozen external drainage pipes can cause a backup of water that eventually triggers the condensate pump alarm.

Typical repair cost £120-280 (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 F.77 fault code

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

Source

Vaillant F.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 27 April 2026