91 High

Navien 91 Fault Code

The Navien 91 fault code means: your boiler's computer has lost its internal settings, meaning it cannot remember how to operate correctly and requires a professional to reset or replace the main control board.. It appears on 1 Navien model in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description: Data loss in EEPROM: Failure in Class B-SW: Irreparable data loss in EEPROM
Typical repair cost £250-450 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Navien models that show the 91 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Navien 91 fault code information on this page is sourced and cross-checked against the official Navien service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026