Navien · Tankless Water Heater Moderate

E351 — Water Flow Sensor Fault

The flow turbine sensor signal is missing or out of range — the unit cannot reliably measure GPM.

Keep using it?
Yes, with caution
DIY difficulty
Moderate
Verified against
navieninc.com ↗

How to fix it

  1. Shut off water supply and close isolation valves.
  2. Clean the cold-water inlet screen.
  3. Remove the flow sensor and check the turbine for free rotation; clear debris.
  4. Reseat the sensor harness at PCB.
  5. Replace the flow sensor if turbine spins freely but no signal is detected.
  6. Descale the unit if hard-water deposits are visible.

Why it happens

Typical signs: E351 on display · Unit does not fire on hot demand · Erratic outlet temperature · Burner won't light despite open tap

  • Flow sensor turbine jammed by debris Sediment or pipe scale stops the impeller from spinning.
  • Failed Hall-effect sensor Sensor electronics no longer outputting a pulse train.
  • Loose or corroded sensor harness Intermittent signal to PCB.
  • Clogged inlet screen Restricted flow keeps the turbine below activation threshold.

Parts you may need

If diagnosis points to a failed part, these are the common replacements. As an Amazon Associate we earn from qualifying purchases.

  • Navien flow sensor Common replacement
    Amazon →

More Navien tankless water heater codes

E003 E003 — Ignition Failure E003C E003C — Cascade Ignition Failure E012 E012 — False Flame / Flame Loss E016 E016 — Overheat / High-Limit Trip E027 Tankless E027 — Sensor Abnormality E030 E030 — Abnormal Exhaust Temperature (Flue Overheat)

All Navien codes → All tankless water heater codes →

Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician. Verified against manufacturer documentation: navieninc.com.