How much does it cost to fix Navien error code E740?

Typical repair cost for the Navien E740 fault is £120-180 including labour.

E740 Low

Navien E740 Fault Code

Q: What does the Navien E740 fault code mean?

Your boiler has lost the connection to the thermometer installed outside your home, so it cannot automatically adjust your heating based on the current weather.

The Navien E740 fault code means: your boiler has lost the connection to the thermometer installed outside your home, so it cannot automatically adjust your heating based on the current weather.. It appears on 7 Navien models in our database.

Technical description: Outdoor sensor error

Typical repair cost £120-180 (including labour)

Engineer required

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

Find a Gas Safe engineer

Search the official register for a qualified engineer in your area.

Gas Safe Register →

Navien models that show the E740 fault code

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

LCB700 Repair: £120-200
NCB Repair: £100-£200
NCB 28 HE Repair: £120-200
NCB 35 HE Repair: £120-200
NCB 40 HE Repair: £120-200
NCB CE Repair: £120-180
NCB H Repair: £120-200

Source

Navien E740 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