E438 Moderate Moderate DIY

Navien Tankless Water Heater E438 — Inlet Thermistor Fault

Navien Tankless Water Heater · Error code E438

What E438 Means

The cold-water inlet thermistor is open, shorted, or reading out of range.

Symptoms

  • E438 on display
  • Erratic outlet temperature
  • Unit will not modulate properly

Common Causes

1
Failed inlet thermistor

Sensor element failed.

2
Bad connection

Loose, corroded, or wet connector.

3
Wiring damage

Harness pinched or chafed.

How to Fix It

  1. Power off the unit.
  2. Reseat the inlet thermistor connector.
  3. Measure resistance against the spec chart at known water temperature.
  4. Replace the thermistor if out of range.
  5. Inspect and repair any damaged harness.

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 inlet thermistor 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.