BoilerFaultCodesThe 6 most common faults
01
F1 High Engineer
Your boiler has automatically turned itself off because it has become too hot to operate safely.
Full guide for F102
F3 High Engineer
Your boiler has lost the ability to accurately measure the temperature of the water flowing through its system, so it has shut down as a precaution to prevent overheating.
Full guide for F303
F4 High Engineer
Your boiler has stopped working because it cannot light its internal flame or needs a simple drain clearance.
Full guide for F404
F5 High Engineer
Your boiler has detected a fault with the sensor that monitors the temperature of the gases leaving your flue, meaning it has switched itself off for safety.
Full guide for F505
F8 High Engineer
Your boiler has detected that the internal fan responsible for venting exhaust gases is not spinning at the required speed, so it has safely shut itself down to prevent unsafe operation.
Full guide for F806
F9 High Engineer
Your boiler has detected that the internal exhaust fan is spinning when it should be turned off, which is preventing the system from safely starting up.
Full guide for F9on the Heatline Sargon 24
Overheat thermostatThermal fusePumpDiverter valveNTC primary sensorWiring harness
Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.
Call a Gas Safe engineer if…
- You can smell gas or see signs of a leak
- The Heatline shows an Emergency or High severity code
- The boiler keeps locking out after repeated resets
- You've tried the DIY checks and the fault hasn't cleared
- There's visible water leaking from the boiler
- The flame is yellow or orange instead of blue