How much does it cost to fix Navien error code E18?

Typical repair cost for the Navien E18 fault is £150-350 including labour.

E18 Emergency

Navien E18 Fault Code

Q: What does the Navien E18 fault code mean?

Your boiler has detected that the temperature of the gases leaving the flue is higher than it should be, so it has safely shut itself down to prevent damage.

The Navien E18 fault code means: your boiler has detected that the temperature of the gases leaving the flue is higher than it should be, so it has safely shut itself down to prevent damage.. It appears on 6 Navien models in our database.

Technical description: Exhaust gas sensor overheat error

Typical repair cost £150-350 (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 E18 fault code

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

NCB Repair: £150-300
NCB 28 HE Repair: £150-350
NCB 35 HE Repair: £150-350
NCB 40 HE Repair: £150-350
NCB H Repair: £150-350
NHB 29 Repair: £150-350

Source

Navien E18 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