125 High

Main 125 Fault Code

The Main 125 fault code means: your boiler is failing to detect the expected temperature rise after the flame ignites, which means it cannot safely heat your water or radiators.. It appears on 1 Main model 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: Primary water temperature has not changed by 1°C within 15-30 seconds of burner lighting or within 10 minutes of burner lighting
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 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 27 April 2026