E594 Serious Moderate DIY

Navien Tankless Water Heater E594 — Gas Valve / Modulation Fault

Navien Tankless Water Heater · Error code E594

What E594 Means

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

Symptoms

  • E594 on display
  • Unit will not fire
  • Inconsistent flame
  • Burner over- or under-firing

Common Causes

1
Failed gas valve coil

Modulation winding open or shorted.

2
Loose wiring at gas valve

Connector partially seated.

3
PCB driver failure

Board not driving the valve correctly.

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.

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 gas valve 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.