Navien E740: outdoor temperature sensor fault

In combination systems, where the Navien heats both water and the home through an air handler, an outdoor sensor tells the system how cold it is outside. E740 means that sensor reading is invalid.

The meaning and recommended action below come from Navien's User's Information Manual for NPE-2 condensing water heaters (NPE-180A2, NPE-210A2, NPE-240A2 and the S2 models). Navien technical support: 1-800-519-8794.

Navien notes this code applies when an air handler unit (AHU) for space heating is connected to the water heater, a setup used in combination water and space heating systems.

What to check

• Look at the outdoor sensor, usually on a north-facing exterior wall, for damage.
• Check that its wire has not been cut or pulled loose, for example by landscaping or siding work.

What a technician does

Tests the sensor and its wiring, and replaces the sensor if it has failed. Hot water heating itself is usually unaffected, but space heating control is.

How to reset it

Navien says you can try resetting the water heater by pressing the Back button on the front panel. If the code returns, the cause is still there and the unit needs service.

Before you call

Have the model number and serial number from the rating plate on the side of the unit ready, along with the date it was installed and the exact code. Navien asks for this information when you contact technical support, and your installer should be your first call for warranty service.

See every code on our Navien error codes page.