740 Medium

Navien 740 Fault Code

The Navien 740 fault code means: your boiler's external temperature sensor is reporting an issue, which may cause it to run inefficiently or struggle to heat your home correctly.. It appears on 7 Navien 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: 740Error
Typical repair cost £120-200 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Navien models that show the 740 fault code

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

Source

Navien 740 fault code information on this page is sourced and cross-checked against the official Navien service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026