104 High

Main 104 Fault Code

The Main 104 fault code means: your boiler has lost communication with an optional safety device, meaning it has shut down as a precaution to prevent unsafe operation.. It appears on 2 Main models 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: errors related to the optional Safety Assist accessory
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.

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 7 July 2026