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, which is usually a flexible silver hose under the boiler.Slowly open the small valve (or valves) on the hose until you hear water moving.Watch the pressure gauge and close the valves tightly once the needle reaches between 1.0 and 1.2 bar.
Your boiler has failed to light a flame, which means you will not have any heating or hot water until the issue is resolved.
Check if other gas appliances in your home (like a hob) are working to ensure gas is reaching the propertyEnsure your gas meter has credit if you use a topped-up prepay meterPress the 'Reset' button on the boiler control panel once to see if the fault clears
Your boiler has overheated, causing a safety switch to shut the system down to prevent damage.
Allow the boiler to cool down for at least 30 minutesCheck that all radiator valves are fully openCheck the boiler pressure is between 1.0 and 1.5 barPress the selector switch to the 'R' (Reset) position for a few seconds
Your boiler is struggling to light or detect a flame, meaning it cannot safely heat your water or radiators.
Check that your gas emergency control valve is open and you have credit on your gas meterTurn other gas appliances like a hob on to confirm gas is flowing to the propertyPress and hold the 'R' reset button for a few seconds to see if the boiler restarts
Your boiler has stopped working because the pipe that removes waste water is blocked, causing water to back up inside the unit.
Check the plastic pipe leading from the boiler to the outside drain for signs of freezing or blockages.If the external pipe is frozen, gently pour warm (not boiling) water over it to melt the ice.Ensure the internal siphon/trap is clear of debris if accessible without opening the sealed boiler casing.Reset the boiler by turning the selector switch to the 'R' position for a few seconds.
Your boiler has detected a problem with the temperature sensor that monitors the water being sent to your taps and radiators, causing it to stop working as a safety precaution.
Full guide for AL 05on the Sime Format DGT HE Combi
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