BoilerFaultCodesThe 6 most common faults
01
Neon 1 not lit High DIY-safe
Your boiler appears to have no power reaching it, which is likely caused by an external electrical issue or a blown fuse inside the unit.
What to try first
Check if your home's main fuse board (consumer unit) has tripped and flip the switch back if necessary.Locate the fused spur switch near the boiler (usually a wall switch with a red light) and ensure it is turned on.Check if other appliances in the house are working to rule out a general power cut.
02
Neon 2 not lit High DIY-safe
Your boiler has stopped working because it has either lost water pressure or a safety sensor has detected that it is getting too hot.
What to try first
Check your pressure gauge located on the boiler; if it is below 1 bar, increase it using the external filling loop until it reaches 1.5 barSwitch the boiler off at the fused spur for 30 seconds and switch it back on to reset the systemEnsure all radiator valves are fully open to allow water to circulate freely
03
Neon 3 not lit High Engineer
Your boiler has stopped working because it cannot confirm that the internal fan is safely clearing waste gases from the system.
Full guide for Neon 3 not lit04
Neon 4 not lit High Engineer
Your boiler is failing to light its pilot light or detect a steady flame, meaning it cannot start the heating process.
Full guide for Neon 4 not lit05
Main burner not operating High Engineer
Your boiler is failing to ignite the main burner because there is an internal electrical connection or control fault preventing it from starting up safely.
Full guide for Main burner not operating06
F1 High Engineer
Your boiler has stopped working because a protective electrical fuse has blown, causing the system to shut down for safety.
Full guide for F1on the Glow-worm Glow-worm Micron 100FF 120FF
PCB FusePrinted Circuit Board (PCB)External Fused SpurOverheat thermostatPressure relief valvePressure sensor
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 Glow-worm 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