How much does it cost to fix Main error code 104?

Typical repair cost for the Main 104 fault is £120-250 including labour.

104 High

Main 104 Fault Code

Q: What does the Main 104 fault code mean?

Your boiler's internal communication system is experiencing a fault with an add-on device, causing the system to stop working as a safety precaution.

The Main 104 fault code means: your boiler's internal communication system is experiencing a fault with an add-on device, causing the system to stop working as a safety precaution.. It appears on 1 Main model in our database.

Technical description: Safety Assist accessory error

Typical repair cost £120-250 (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 104 fault code

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

Insul 7 Repair: £120-250

Source

Main 104 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