BoilerFaultCodesThe 6 most common faults
01
0.5bar High DIY-safe
Your boiler has stopped working because the water pressure inside the heating system has dropped too low.
What to try first
Locate the filling loop (usually a silver braided hose with one or two valves) located underneath your boiler.Slowly open the valve(s) to let water into the system until the pressure gauge on the front of the boiler reads between 1.0 and 1.5 bar.Close the valve(s) firmly and ensure no water is leaking from the connection points.
02
F3 High Engineer
Your boiler is struggling to circulate water, which means it cannot safely heat your home or provide hot water.
Full guide for F303
01 High Engineer
Your boiler is failing to light because it cannot detect a flame, meaning you currently have no heating or hot water.
Full guide for 0104
02 High Engineer
Your boiler is failing to clear used air and gases because the internal extraction system is not starting, meaning the boiler has safely locked out to prevent unsafe operation.
Full guide for 0205
12 High Engineer
Your boiler's internal air extraction system is not running at the correct speed, which has caused the boiler to shut down for safety reasons.
Full guide for 1206
13 High Engineer
Your boiler's internal computer has encountered a serious communication error and has shut down for safety, meaning you currently have no heating or hot water.
Full guide for 13on the Glow-worm CXi / CSi
Filling loopPressure relief valveExpansion vesselCentral heating pumpPrinted Circuit Board (PCB)Ignition electrode
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