303 High

Navien 303 Fault Code

The Navien 303 fault code means: your boiler's internal management system has detected a critical communication error with the main brain of the unit, causing it to shut down for safety reasons.. 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: 303:Eng head t’ couple cont: A failure of the engine head control thermocouple was detected.
Typical repair cost £200-450 (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 303 fault code

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

Source

Navien 303 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