BoilerFaultCodesMain 25 Fault Code
The Main 25 fault code means: your boiler has effectively switched itself off because it has detected it is getting too hot, likely due to a circulation or blockage issue.. It appears on 3 Main models in our database.
Technical description: Safety Thermostat Activated (Boiler or Flue)
DIY fix possible
Check that your home's radiator valves are fully open.Ensure the boiler pressure is between 1 and 1.5 bar on the gauge.Attempt to reset the boiler using the selector switch.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Main models that show the 25 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Main 25 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