BoilerFaultCodesThe 6 most common faults
Your boiler has stopped working because the water pressure inside the system has dropped too low and needs to be topped up.
Locate the filling loop, which is usually a silver flexible hose with one or two small taps.Slowly open the tap(s) until you hear water flowing and watch the pressure gauge on the boiler panel.Close the taps tightly once the needle reaches between 1.0 and 1.2 bar.
Your boiler is not detecting a flame, which means it cannot light itself to provide heating or hot water.
Check if other gas appliances in your home, such as a hob, are working to confirm gas supply.Ensure your gas meter has credit if you use a prepay meter.Press the 'Reset' button on the boiler control panel to see if it restarts.
Your boiler has overheated, causing a safety sensor to shut the system down to prevent damage.
Check that all radiator valves are fully openBleed all radiators to remove trapped air from the systemOnce the boiler has cooled down, press the 'R' button to reset the unit
Your boiler has detected that the waste water pipe or internal drainage trap is blocked, preventing the unit from draining properly.
Check the white plastic pipe leading outside to see if it is frozen or blocked by debris.If frozen, carefully pour warm (not boiling) water over the external pipe to thaw it.Inspect the internal condensate trap (siphon) if accessible and clean it of any visible sludge or debris.Reset the boiler once the blockage is cleared.
Your boiler has detected a problem with the temperature sensor that monitors the water being sent to your radiators, causing the system to stop working for safety.
Full guide for AL 05Your boiler is struggling to light or detect a flame, which means it will shut down to prevent wasting gas.
Full guide for AL 08on the Sime Format DGT HE System
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