F.042 High

Vaillant F.042 Fault Code

Also written as F042 — same fault.

The Vaillant F.042 fault code means: your boiler has lost the electrical connection to the part that manages the gas supply, meaning it cannot safely start up and will stop providing heat and hot water.. It appears on 3 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: Interruption in the wiring harness to the gas valve assembly. The coding resistor (in the wiring harness) or the gas group resistor is invalid.
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.042 fault code

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

Source

Vaillant F.042 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