F.43 High

Vaillant F.43 Fault Code

Also written as F43 — same fault.

The Vaillant F.43 fault code means: your boiler has detected an internal wiring problem or a connection issue that is preventing it from running safely.. It appears on 9 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.43 fault?

This fault occurs when the boiler's internal computer detects a mismatch or a break in the electrical wiring harness that connects the main components. The most common reason is that a wire has become loose, damaged, or was incorrectly connected during a previous repair, preventing the system from communicating safely. In some cases, it can also be caused by using an incorrect wiring loom that doesn't match the specific model of your boiler.

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.43 fault code

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

Source

Vaillant F.43 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