How much does it cost to fix Main error code 321?

Typical repair cost for the Main 321 fault is £150-300 including labour.

321 High

Main 321 Fault Code

Q: What does the Main 321 fault code mean?

Your boiler's internal computer is unable to talk to its control panel, meaning the system has safely shut down to prevent incorrect operation.

The Main 321 fault code means: your boiler's internal computer is unable to talk to its control panel, meaning the system has safely shut down to prevent incorrect operation.. It appears on 1 Main model in our database.

Technical description: Communication error

Typical repair cost £150-300 (including labour)

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 321 fault code

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

Insul 7 Repair: £150-300

Source

Main 321 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 27 April 2026