Navien E740 Error Code — What It Means
Error code E740 on a Navien tankless water heater indicates an abnormality in the outdoor temperature sensor. This code appears only when the outdoor reset curve feature is enabled on your system. The outdoor reset function adjusts water temperature based on outside air temperature, so the control board flags E740 when it cannot read a valid signal from the outdoor sensor.
The error typically points to a configuration mistake, a loose or corroded sensor connection, or a failed outdoor temperature sensor itself. It is most relevant to systems using outdoor reset control logic or hydronic heating applications, rather than simple domestic hot water setups.
Common Causes
- Outdoor reset curve not enabled or misconfigured The control settings do not match the installed sensor or outdoor reset is turned off when a sensor is connected.
- Loose or corroded sensor wiring connection The plug at the sensor or at the main control board has worked loose, become corroded, or has damaged terminals.
- Damaged outdoor temperature sensor cable The wire run between the sensor and the unit is cut, pinched, or shorted by moisture or rodent damage.
- Failed outdoor temperature sensor The sensor element itself is open or out of specification and no longer sends a valid resistance signal.
- Main control board input circuit failure The sensor and wiring are good but the board cannot process the signal due to a failed input channel or circuit damage.
Step-by-Step Fix
- Verify model and outdoor reset application. Confirm that your Navien system is configured for outdoor reset control and that the outdoor sensor is required for your installation.
- Check outdoor reset configuration in the control menu. Access the service settings and verify that the outdoor reset curve is enabled and correctly programmed for your model and application.
- Inspect the outdoor sensor wiring. Follow the cable from the outdoor sensor to the main board and check every plug, terminal, and wire segment for looseness, corrosion, physical damage, or moisture intrusion.
- Test the outdoor temperature sensor electrically. Disconnect the sensor and measure its resistance, then compare the reading with the factory specification table for your exact model to confirm the sensor is within range.
- Replace the outdoor temperature sensor if it is out of specification. Install a new sensor in the same mounting location, route the cable along the original path, and plug it securely into the control board connector.
- Inspect the control board harness and connectors. If the sensor tests good but the code persists, check the board-side connector for bent pins, corrosion, or evidence of moisture, and evaluate the main control board for input circuit failure.
- Power-cycle the unit and clear the error. Turn off power at the breaker for thirty seconds, restore power, and verify that E740 clears and the outdoor reset function responds normally to temperature changes.
Parts Often Needed
| Part | Notes |
|---|---|
| Navien outdoor temperature sensor | Amazon | Match the part number to your exact Navien model and year for correct resistance curve. |
| Outdoor sensor wiring harness or connector kit | Amazon | Order if the cable or plug is damaged and cannot be repaired in place. |
| Navien main control board | Amazon | Required only if the sensor circuit tests good but the board cannot read the signal. |
When to Call a Pro
Call a licensed heating technician if you are not comfortable working with low-voltage control wiring or if you do not have a multimeter and the factory resistance table for your model. A qualified service provider can quickly test the sensor circuit, verify outdoor reset programming, and replace the sensor or control board under warranty if applicable. Professional diagnosis is also recommended if the error returns after you have checked wiring and replaced the sensor, since persistent E740 may indicate a control board failure or a more complex system integration issue.