Navien NPE-2 Code E594
EEPROM fault. The memory that holds the heater's stored settings is not reading back correctly. This code exists on the NPE-2 and not on the original NPE, whose manual has no E594 at all. Navien's instruction is to contact technical support.
Call a proThe verdict
Call a licensed plumber
Stored configuration that cannot be trusted is a board-level problem. It generally ends in a control board replacement, with the unit's settings entered again afterwards.
Typical cost: $200–$1,100
US estimate for parts and labor. Prices vary by region and by who does the work — see how we estimate costs.
What the manual says
The meaning above is transcribed from the manufacturer's diagnostic documentation for the NPE-2 — not averaged across the brand.
Source: Navien NPE-2 User's Information Manual (p. 17), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
- Note any recent service, board replacement, or power event.
- Check whether the unit is still within its parts warranty before booking.
- Contact your installer or Navien technical support.
Frequently asked questions
What is stored in an EEPROM on a water heater?
The settings that have to survive a power cut: model configuration, gas type, installation options, service counters. Which is why an unreadable one is a board problem rather than a setting you can simply re-enter.