BoilerFaultCodesThe 6 most common faults
Your boiler has stopped working because the water pressure is too low and it needs to be topped up.
Locate the filling loop (usually a silver braided hose with small valves) underneath the boiler.Slowly open the valves one at a time until you hear water flowing.Watch the pressure gauge on the front of the boiler until it reaches between 1.0 and 1.2 bar, then close both valves tightly.
Your boiler has tried to light but cannot detect a flame, meaning it won't produce any heat or hot water.
Check if other gas appliances (like a hob) are working to ensure gas is reaching your home.Verify that your gas prepaid meter hasn't run out of credit.Press the 'Reset' button on the boiler control panel once to see if the system restarts.
Your boiler has switched itself off because it has detected that the water inside the unit is getting too hot.
Check that your radiator valves are open and not turned off.Verify that your system water pressure is between 1 and 1.5 bar on the gauge.Try to reset the boiler by turning the selector dial to the 'OFF/RESET' position and then back to 'ON'.
Your boiler is failing to detect a flame, which means it cannot fire up to provide heating or hot water.
Check that your gas emergency control valve is open and other gas appliances are working.Reset the boiler by turning the selector switch to the 'R' position for a few seconds.Verify if your pre-paid gas meter has credit, if applicable.
Your boiler has detected a fault with the internal temperature sensor that monitors heat being sent to your taps or radiators, preventing the system from heating up.
Full guide for AL 05Your boiler is unable to control the gas flow properly because a vital internal electrical connection has been lost, causing the system to shut down for safety.
Full guide for AL 11on the Sime Metropolis DGT 25 BF
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