How much does it cost to fix Navien error code E777?

Typical repair cost for the Navien E777 fault is £150-300 including labour.

E777 High

Navien E777 Fault Code

Q: What does the Navien E777 fault code mean?

Your boiler has detected that a safety device installed outside the main unit is reporting a dangerously low water level, preventing the boiler from starting to protect it from damage.

The Navien E777 fault code means: your boiler has detected that a safety device installed outside the main unit is reporting a dangerously low water level, preventing the boiler from starting to protect it from damage.. It appears on 4 Navien models in our database.

Technical description: LWCO (service code: 7700) (external device)

Typical repair cost £150-300 (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 E777 fault code

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

Source

Navien E777 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