E351 Moderate Moderate DIY

Navien Tankless Water Heater E351 — Water Flow Sensor Fault

Navien Tankless Water Heater · Error code E351

What E351 Means

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

Symptoms

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

Common Causes

1
Flow sensor turbine jammed by debris

Sediment or pipe scale stops the impeller from spinning.

2
Failed Hall-effect sensor

Sensor electronics no longer outputting a pulse train.

3
Loose or corroded sensor harness

Intermittent signal to PCB.

4
Clogged inlet screen

Restricted flow keeps the turbine below activation threshold.

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.

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.

Common replacement Navien flow sensor View on 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 E027 — Gas Type Setting Error E030 E030 — Fan / Combustion Blower Fault

All Navien codes → All tankless water heater codes →

Verified against manufacturer documentation: navieninc.com

Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician.