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Navien · Boiler

E740 Outdoor sensor not detected

Medium severity Navien NHB-HNavien NFB-H

What the control is telling you

Outdoor reset is enabled but the outdoor sensor is not read. Manual action: check sensor connections.

Applies to: Navien NHB-H (NHB-055H/080H/110H/150H) and NFB-H fire-tube (NFB-175H/200H) condensing boilers

Likely causes

  • Outdoor sensor not installed while Outdoor Reset is enabled (default Enabled)
  • Broken or spliced wire
  • Failed sensor

Test and repair steps

  1. Check the outdoor sensor wires at the OUTDOOR terminal and at the sensor.
  2. Use 18 gauge thermostat wire with no splices, routed away from line voltage.
  3. Check Outdoor Temp in Status Info.
  4. Outdoor reset must stay enabled with a sensor fitted for efficiency regulations; disable it only if the boiler is not used for space heating.

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Source: Navien NHB-H Installation and Operation Manual, 11.14 Understanding Error Codes, p.119; Navien NFB-H Installation Manual, 11.7 Understanding Error Codes, p.116; NHB-H User's Information Manual (V1.0 Apr 2024), 5.2, p.24. Summarized in our own words; check the original manual for your model.

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For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.