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Navien · Tankless water heater

E439 Flow sensor abnormal

Medium severity Navien NPE-2

What the control is telling you

The water flow sensor signal is abnormal.

Applies to: Navien NPE-A2 (built-in recirculation) and NPE-S2 condensing tankless water heaters: NPE-150S2, NPE-180A2/S2, NPE-210A2/S2, NPE-240A2/S2

Likely causes

  • Flow sensor or wiring fault
  • Debris at the inlet filter

Test and repair steps

  1. Clean the cold water inlet filter.
  2. Check the flow sensor connector and wiring.
  3. Compare Status Information > 3. Flow Rate with actual flow.
  4. Reset once with the Back button on the front panel and note whether the code returns.
  5. Fix given phone Navien Technical Support, 1-800-519-8794.

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Source: Navien NPE-A2/S2 User's Information Manual (20049624A), 5.2 Understanding Error Codes, p.17. Summarized in our own words; check the original manual for your model.

Homeowner looking at this code? See what it means for you and when to call a technician.

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.