Navien NCB300 Fault codes & diagnostics

55 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.

Gas 2024-present
GC Numbers 47-709-1747-709-17
Download the Navien NCB300 manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

The fault code is already on your display

Note it down before pressing reset — resetting clears the code immediately. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
55 Unrated
Versions & GC numbers

Which version do you have?

The GC number on your boiler's data badge identifies the exact appliance and production years, as recorded in the UK Product Characteristics Database.

GC number Appliance (register name) Produced
47-709-17 NCB300 NCB300-30K 2024–present
47-709-17 NCB300 NCB300-30K 2024–present

Appliance names and production years are taken from the UK Product Characteristics Database (the SAP/SEDBUK register). Where a GC number is also printed in this model's own manufacturer manual, both sources were checked to agree.

Just want the most common Navien NCB300 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 55 documented codes

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