F.343 High

Vaillant F.343 Fault Code

Also written as F343 — same fault.

The Vaillant F.343 fault code means: your boiler has lost the connection to the internal sensor that measures the air flow needed for safe combustion, causing the unit to stop working as a precaution.. 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: Plug for air-mass flow sensor loose/not plugged in. The signal for the air-mass flow sensor is not plausible.
Typical repair cost £120-250 (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.343 fault code

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

Source

Vaillant F.343 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