BoilerFaultCodesThe 6 most common faults
01
F1 High DIY-safe
Your boiler has stopped working because the water pressure inside the system has dropped too low to operate safely.
What to try first
Locate the external filling loop, which is a silver braided hose connecting your cold mains pipe to your heating pipework.Open the valves at both ends of the hose to allow mains water to enter the system until the pressure gauge reads between 1.0 and 1.5 bar.Carefully close both valves completely and ensure the hose remains attached but tightly shut.
02
FU High DIY-safe
Your boiler has detected that the water inside it is getting dangerously hot because it cannot circulate through your radiators effectively.
What to try first
Check that the radiator valves on your radiators are open.Ensure the isolation valves underneath the boiler pipes are in the open position.Check if your central heating pump is running or has become seized.
03
L1 High Engineer
Your boiler has shut itself down because the water inside is either getting too hot or is not circulating properly.
Full guide for L104
L2 High Engineer
Your boiler has failed to light the flame, so it has safely shut itself down to prevent a gas build-up.
Full guide for L205
F2 High Engineer
Your boiler has successfully lit but has suddenly lost its flame, meaning it cannot heat your home or water.
Full guide for F206
Fn High Engineer
Your boiler has lit up, but it cannot keep the flame going, so it has safely shut itself down to prevent gas from escaping.
Full guide for Fnon the Keston System 30
Pressure relief valvePressure sensorExpansion vesselCentral heating pumpFlow / Return thermistorsHeat exchanger
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 Keston 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