Navien NFB-C E740 — Outdoor Temperature Sensor
E740
Outdoor temperature sensor. When the outdoor sensor reads out of the normal range (under 2.2 kΩ or over 122.2 kΩ) for three seconds and the outdoor reset curve is enabled, the boiler switches from reset curve mode to normal mode.
Applies to
This sensor fault code is displayed by the front-panel display on NFB-C models (NFB-301C, NFB-399C), as documented in NFB-C Installation and Operation Manual, Ver 1.0 (Feb 2024) — error codes cross-checked against the NFB-C Service Manual (250116) (Ver 1.0). Codes can differ between controller generations, so always check the manual that matches your unit.
Controller: front-panel display. Source manual: Ver 1.0 (2024-02).
Why a code is not a diagnosis
A sensor code usually means the sensor, its wiring or its mounting is faulty; the physical condition the sensor measures (temperature, pressure, flow) may be perfectly normal.
Safe things to observe
- Note the code; there is no homeowner-accessible check for sensor circuits.
- If the appliance still runs, watch whether the code returns and report it when you call.
Do not
- Do not open the appliance to test or replace sensors yourself.
How professional service helps
- A technician measures the sensor and harness against the manual's values and replaces the faulty component.
- Combustion and safety devices are verified with an analyser and the manufacturer's procedure after any repair.
Where ROMA provides on-site service
On-site service is currently offered in Greater Vancouver, BC, including Vancouver, Burnaby, Richmond, Surrey, Coquitlam, North Vancouver, West Vancouver, New Westminster, Port Coquitlam, Port Moody, Delta, Langley, Maple Ridge, Abbotsford, Chilliwack, White Rock, Pitt Meadows, Tsawwassen, Anmore, Lions Bay, Belcarra, Bowen Island, Mission. Everything else on this site is reference information you can use anywhere in Canada.
Parts and service
Parts commonly involved
These parts appear in the documented text for this alert. A code reports a condition, not a failed part — a technician confirms the cause by testing before anything is replaced.
Outdoor reset sensor
An outdoor reset sensor reads the outside temperature so a boiler's control can match the water temperature it sends out to the weather — milder days…
What a technician checks →Boiler sensors (thermistors)
Boiler sensors are small temperature probes, usually thermistors, that tell the control how hot the water and flue gases are so it can fire safely…
What a technician checks →
Service for this alert
- Gas Boiler Repair
Licensed gas technicians from ROMA Heating & Cooling diagnose and repair residential and light-commercial gas boilers across…
Every boiler part explained — what it does, how it fails and what a technician checks.
Documented for Navien NFB-C Commercial Boiler.
Frequently asked questions
Can I clear this code myself?
Clearing a code without fixing its cause usually just brings it back. Note the code, check the safe items on this page, then call a licensed technician. If your appliance manual describes a user reset for this exact code and it stays away, no call may be needed.
Boiler locked out? Call with the alert text ready.
The brand, model and this alert are passed along when you request service online, so the technician arrives prepared.
On-site service in Greater Vancouver, BC. ROMA lists “Open 7 days” on its site; call to confirm same-day availability.
Sources and verification
- NFB-C Installation and Operation Manual, Ver 1.0 (Feb 2024) — error codes cross-checked against the NFB-C Service Manual (250116) · Ver 1.0 · accessed September 7, 2026
Facts checked September 7, 2026. Last updated September 7, 2026. Technical reviewer: not yet assigned.