BoilerFaultCodesThe 5 most common faults
01
Red LED on (solid) High DIY-safe
Your boiler has stopped working because it cannot detect a flame, likely due to a temporary interruption in the gas supply or a part that needs cleaning.
What to try first
Check that your gas meter is in credit and the gas emergency control valve is openCheck if other gas appliances in your home, like a hob, are working correctlyPress and hold the reset button for a few seconds to see if the boiler restarts
02
Red LED after flow switch High DIY-safe
Your boiler has detected that water is not circulating correctly or is getting too hot, so it has shut down to prevent damage.
What to try first
Check that your boiler pressure is between 1 and 1.5 barEnsure all radiator valves and external isolation valves are fully openSwitch the boiler off and back on again to see if the fault clears
03
Red LED on (after safety stat) High Engineer
Your boiler has shut down because it has detected an unsafe level of heat or a problem with how waste gases are being vented.
Full guide for Red LED on (after safety stat)04
LED flashing after fan High Engineer
Your boiler has stopped working because the internal fan, which safely exhausts gases, is not starting or running correctly.
Full guide for LED flashing after fan05
Orange LED flashing High Engineer
Your boiler has detected a problem with the temperature sensor that manages your central heating, meaning it cannot accurately monitor the water temperature.
Full guide for Orange LED flashingon the Sime Friendly
Ignition electrodeFlame sensing probeGas valveCirculating pumpWater flow switchOverheat limit thermostat
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