100mm Emergency

Main 100mm Fault Code

Also written as 100MM — same fault.

The Main 100mm fault code means: your boiler has detected that the flue pipe is set to the wrong size or is incorrectly configured for your specific model, which prevents it from safely venting exhaust gases.. It appears on 1 Main model in our database.

Act immediately

Turn off your boiler and stop using gas appliances. Call a Gas Safe engineer now. This fault can be dangerous if ignored.

Technical description: Flue Terminal Diameter
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 100mm fault code

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

Source

Main 100mm 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