304 Low

Main 304 Fault Code

The Main 304 fault code means: your boiler is currently running an internal software adjustment process, which usually happens after maintenance or a power reset.. It appears on 1 Main model in our database.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: Calibration Mode

Engineer required

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

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Main models that show the 304 fault code

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

Source

Main 304 fault code information on this page is sourced and cross-checked against the official Main service / installation manuals for each affected model.

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