BoilerFaultCodesMain 125 Fault Code
The Main 125 fault code means: your boiler is failing to detect the expected rise in temperature after the burner fires up, indicating it cannot effectively transfer heat into your system.. It appears on 6 Main models in our database.
Technical description: primary water temperature has not changed by 1° C within 15-30 seconds of burner lighting, or if within 10 minutes of burner lighting, the primary water temperature has not changed
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 125 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Main 125 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