Navien E740 Fault Code: Causes, Fixes & Repair Costs
What does the Navien E740 fault code mean?
The E740 code signals that the boiler has detected an abnormal reading from the outdoor temperature sensor — either an open circuit (no signal) or a short circuit (incorrect signal). This alarm only activates when the outdoor reset curve (weather compensation) feature is enabled in the boiler's settings. The outdoor reset curve allows the boiler to modulate its output based on external temperatures, improving efficiency. When the sensor feeding this function fails or loses its connection — or when the feature is switched on without a sensor physically installed — the E740 alarm is triggered. Importantly, this is not a gas or combustion fault, so heating may continue to operate, but weather compensation will be impaired and efficiency reduced.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Outdoor reset curve enabled without a sensor fitted Common
The most frequent trigger: someone has accidentally switched on the outdoor reset curve in the boiler's settings menu, but no physical outdoor sensor is actually connected. The boiler expects a sensor signal, finds none, and raises E740. Disabling the feature in the service menu clears the fault immediately.
- Loose or disconnected sensor wiring Common
The outdoor temperature sensor is mounted externally and its wiring runs back to the boiler. Vibration, weathering, or accidental disturbance can cause the cable to pull away from its connector, giving the boiler an open-circuit reading and triggering E740.
- Damaged sensor cable Sometimes
Exterior wiring is exposed to UV, frost, and pests. A cracked, pinched, or rodent-damaged cable can cause an intermittent or permanent short or open circuit, producing the same E740 alarm even if the sensor itself is healthy.
- Failed outdoor temperature sensor Sometimes
The sensor itself has developed an internal open or short circuit. This is an inexpensive component but will need replacement by a Gas Safe engineer once wiring has been ruled out as the cause.
- PCB or configuration fault Rare
In rare cases, the control board misreads the sensor circuit even when all wiring and the sensor are sound. This is uncommon and typically only suspected once all other causes have been eliminated.
How to fix it
- Check whether an outdoor sensor is physically fitted DIY safe
Look outside the property for a small sensor unit (usually a white or grey plastic housing) mounted on an external wall, typically on a north-facing or shaded surface. If no sensor is present, the outdoor reset curve has likely been enabled by mistake — a Gas Safe engineer can access the service menu to disable it.
- Attempt a boiler reset DIY safe
Press the Back button on the Navien front panel to reset the boiler and clear the alarm. If the E740 returns immediately or within a short time, a reset alone will not solve the underlying issue. Do not reset more than two or three times.
- Visually inspect the external sensor cable DIY safe
Without opening any boiler panels or touching any electrical terminals, trace the cable from the outdoor sensor back to where it enters the building. Look for obvious physical damage — kinks, splits, bite marks, or the cable having pulled free from a weatherproof gland. Do not attempt to open connectors or probe the wiring yourself.
- Have a Gas Safe engineer check the sensor settings and wiring connections Gas Safe engineer
A registered engineer will access the Navien service menu to verify whether the outdoor reset curve is correctly configured and confirm a sensor is present. They will also check resistance values across the sensor and wiring to determine whether the fault lies in the cable, the connector, or the sensor itself.
- Sensor or wiring replacement if required Gas Safe engineer
If the engineer finds a broken cable, a failed connection, or a sensor that has gone open or short circuit internally, they will replace the faulty component. The outdoor temperature sensor is an inexpensive part; the labour to fit and reconfigure it makes up the bulk of the cost.
- Call a Gas Safe registered engineer if the fault persists or you are unsure Gas Safe engineer
If the E740 returns after a reset, if you cannot identify a sensor on your property, or if any internal boiler work is needed, contact a Gas Safe registered engineer. You can verify an engineer's credentials at gassaferegister.co.uk.
Parts you may need
- Navien outdoor temperature sensor · from £25
- Outdoor sensor cable/extension wiring · from £15
- Weatherproof cable gland/entry fitting · from £5
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £100–£250, depending on the underlying cause.
Frequently asked questions
Will my heating still work with the E740 fault showing?
In most cases, yes — E740 is a weather compensation alarm rather than a lockout or combustion fault, so the boiler will often continue to provide heating and hot water. However, the outdoor reset function will be disabled, meaning the boiler loses its ability to modulate output based on external temperature. This can reduce efficiency and increase fuel costs until the fault is resolved.
I haven't had an outdoor sensor fitted — why is E740 appearing?
This is the most common scenario. The outdoor reset curve feature may have been switched on in the boiler's service settings — either during a previous service visit or by accident. Because the boiler expects a sensor signal and finds none, it raises E740. A Gas Safe registered engineer can access the service menu and disable the outdoor reset curve, which will clear the fault without any parts being needed.
How much does it cost to fix the Navien E740 fault?
Most repairs fall in the region of £100–£250 including parts and labour. A wiring reconnection or cable repair tends to sit at the lower end, while a full outdoor sensor replacement with reconfiguration sits toward the upper end. If the fault turns out to be a PCB issue — which is uncommon — costs could rise to £300–£450, but this is rarely the cause of E740.
Can I fix the E740 fault myself?
A homeowner can safely check whether an outdoor sensor is physically present, visually inspect the external cable for obvious damage, and attempt a boiler reset. However, accessing the service menu to change the outdoor reset curve settings, testing wiring resistance, or replacing any components must be done by a Gas Safe registered engineer. Carrying out gas appliance work without Gas Safe registration is illegal in the UK and can invalidate your boiler warranty.