N.343 Medium

Vaillant N.343 Fault Code

Also written as N343 — same fault.

The Vaillant N.343 fault code means: your boiler is struggling to accurately measure the air it needs for combustion, so it is running in a limited state to remain operational.. It appears on 3 Vaillant models in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: The signal for the air-mass flow sensor is not plausible. If the sensor set for the air-mass flow is connected and there is feedback, but the values are implausible, the unit works in limp home mode.
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 N.343 fault code

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

Source

Vaillant N.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