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Navien · Combi boiler

E740 Outdoor sensor not detected

Low severity Navien NCB-HNavien NFC-H

What the control is telling you

Outdoor reset is enabled but the outdoor temperature sensor is not read. NFC-H: appears only when the outdoor reset curve is enabled. The boiler then runs on the set space heating temperature. NFC-H manual lists the reset type as: Alarm.

Applies to: Navien fire-tube combi boilers, NFC-H series (NFC-250/200H, NFC-250/175H) and NCB-H combi boilers (NCB-250/150H, NCB-240/130H, NCB-240/110H, NCB-190/080H, NCB-190/060H)

Likely causes

  • Outdoor sensor not installed
  • Open or loose sensor wiring
  • Outdoor reset enabled by mistake

Test and repair steps

  1. Make sure the outdoor reset curve is set up correctly (the manual's listed remedy).
  2. Check the outdoor sensor wiring at the terminal block; use 18 gauge thermostat wire with no splices, routed away from line voltage.
  3. Check Status Information > 13. Outdoor Temp.
  4. If no sensor is installed, set Space Htg Operation > 3. Outdoor Reset ON/OFF to Disable.

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Source: Navien NCB-H Installation & Operation Manual, 11.7 Understanding Error Codes, p.129-130; Navien NFC-H Installation Manual, 11.7 Understanding Error Codes, p.127-128. Summarized in our own words; check the original manual for your model.

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