F.42 High

Vaillant F.42 Fault Code

Also written as F42 — same fault.

The Vaillant F.42 fault code means: your boiler has detected a problem with the internal wiring that sends signals between its control components, meaning it cannot operate safely and has shut itself down.. It appears on 8 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: Faulty cable loom Faulty or incorrect loom fitted
Typical repair cost £150-250 (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 F.42 fault code

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

Source

Vaillant F.42 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