F.42 High

Vaillant F.42 Fault Code

Also written as F42 — same fault.

The Vaillant F.42 fault code means: your boiler has stopped working because of a wiring issue or an electrical connection problem inside the main control panel.. It appears on 18 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

What causes the Vaillant F.42 fault?

This fault occurs when the boiler's internal computer stops receiving the correct electrical signals from the main wiring system. The most common reason is that the plastic connectors have vibrated loose over time or the wires themselves have developed a break or short circuit. Because the control board cannot safely communicate with the various parts of the boiler, it shuts down as a safety precaution.

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.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 27 April 2026