BoilerFaultCodesThe 6 most common faults
Your boiler has stopped working because the water pressure is too low and needs to be topped up.
Locate the filling loop (the silver braided hose) underneath the boiler.Slowly open the small tap(s) on the filling loop until you hear water entering the system.Watch the pressure gauge until it reaches between 1.0 and 1.2 bar, then close the tap(s) fully.
Your boiler is failing to light or stay lit because it cannot detect a flame.
Check if other gas appliances, like a cooker, are working to confirm your gas supply is onEnsure your gas meter has credit if you use a prepay meterTry resetting the boiler by turning the selector switch to the 'R' position for a few seconds
Your boiler has overheated, prompting a safety device to shut it down to prevent damage.
Check that all radiator valves are fully openCheck the system pressure is between 1.0 and 1.5 barBleed any trapped air from your radiatorsAttempt to reset the boiler using the selector switch
Your boiler is struggling to detect a flame, which means it cannot stay lit to provide heating or hot water.
Check that your gas emergency control valve is in the 'on' positionVerify if other gas appliances in your home, such as a hob, are working correctlyPress the reset button on the boiler control panel
Your boiler has detected a fault with the main temperature sensor that monitors the water leaving the unit, preventing it from heating up correctly.
Full guide for AL 05Your boiler's computer is confused about how the internal pipework is set up because an optional secondary sensor is missing or malfunctioning.
Full guide for AL 10on the Sime Meridian HE
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 Sime 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