Navien · Tankless Water Heater Serious

E594 — Gas Valve / Modulation Fault

The gas valve modulation feedback does not match the commanded firing rate — valve coil resistance or position is out of spec.

Keep using it?
Limited use only
DIY difficulty
Moderate
Verified against
navieninc.com ↗

How to fix it

  1. Power off; reseat the gas valve connector.
  2. Measure coil resistance per service-manual spec; replace gas valve if out of range.
  3. If valve coil tests good, replace the PCB.
  4. After any gas valve work, verify manifold pressure and combustion with a combustion analyzer.

Why it happens

Typical signs: E594 on display · Unit will not fire · Inconsistent flame · Burner over- or under-firing

  • Failed gas valve coil Modulation winding open or shorted.
  • Loose wiring at gas valve Connector partially seated.
  • PCB driver failure Board not driving the valve correctly.

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.

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.