Abnormal operation: Outdoor temperature sensor Auto
The E740 error code on Navien boilers indicates abnormal operation: outdoor temperature sensor auto. Symptoms include system displays the error message e740 on the front panel. if this occurs, the boiler changes the control mode.
Navien E740 means: abnormal operation: outdoor temperature sensor auto. The usual causes are thermistor (ntc sensor) failure — open circuit or short circuit, loose or corroded wiring connection at the sensor or sensor out of calibration giving incorrect readings. A Gas Safe registered engineer should diagnose the root cause.
What Does E740 Mean
The E740 code is shown by the boiler’s control board when it detects abnormal operation: outdoor temperature sensor auto. The boiler then stops instead of continuing to run in that condition: the code is the appliance reporting a problem, not proof that the boiler is broken.
| What you see on the boiler |
|---|
| system displays the error message E740 on the front panel. If this occurs |
| the boiler changes the control mode |
Common Causes
Causes documented in 2 technical sources.
| Cause |
|---|
| Thermistor (NTC sensor) failure — open circuit or short circuit |
| loose or corroded wiring connection at the sensor |
| sensor out of calibration giving incorrect readings |
| water damage to the sensor or its wiring |
| PCB input fault — the connector on the PCB may be damaged |
Safety Warning
E740 is a gas-related fault. Never attempt to repair or adjust the gas valve, burner assembly, or flue system yourself. All gas work must be carried out by a Gas Safe registered engineer (UK) or licensed HVAC technician (US). Repeated resets after ignition failure can cause unburnt gas to accumulate inside the combustion chamber — this is a serious safety hazard. If you smell gas at any point, turn off the gas supply at the meter, open windows, leave the property, and call your gas emergency service immediately. Warning: this fault code may relate to gas supply, flue or combustion issues. Do not attempt repair — contact a Gas Safe registered engineer immediately.
Parts & Prices
Prices shown are indicative. Actual prices and availability vary by region.
Step-by-Step Diagnosis
Identify which sensor is faulty from the error code (flow, return, DHW, flue gas, or outdoor)
Check the wiring — inspect the connections at both the sensor and the PCB for corrosion or loose terminals
Measure the sensor resistance with a multimeter — compare with the manufacturer's resistance/temperature chart (typically 10kΩ at 25°C (77 °F) for NTC sensors)
If resistance is out of range (infinite = open circuit, near zero = short circuit), replace the sensor
If resistance is correct, check the wiring continuity between the sensor and PCB
If wiring is good, test the PCB input
How to Fix It
Replace the faulty thermistor/NTC sensor
repair or replace damaged wiring
clean corroded connectors
if the PCB input is damaged, replace the PCB
Refer to the Navien service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
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Affected Models
Navien models that can display fault code E740:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Navien models | — | Possible |
Frequently Asked Questions
Replace the faulty thermistor/NTC sensor. According to the technical documentation, this is the first check on the list for E740.
E740 is classified as a high-severity fault: it involves gas, combustion or flue components. Do not keep resetting the boiler and do not attempt the repair yourself — call a Gas Safe registered engineer.