BoilerFaultCodesThe 5 most common faults
01
Red LED on (solid) High DIY-safe
Your boiler has stopped working because it couldn't successfully light the flame to provide heat or hot water.
What to try first
Check that your gas emergency control valve is in the 'on' positionIf you have a prepay gas meter, ensure it has creditTurn the selector dial to the 'R' (Reset) position for a few seconds, then back to the heating or hot water setting
02
Red LED after flow switch High DIY-safe
Your boiler has detected that water is not circulating properly through the system, causing it to overheat and shut down for safety.
What to try first
Check that your boiler pressure is between 1.0 and 1.5 barEnsure all radiator valves and internal stop taps are fully openGently bleed your radiators to remove any trapped air pockets
03
Red LED on (after safety stat) High Engineer
Your boiler has shut down because it has detected internal overheating or a potential issue with the exhaust gases.
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 clears waste gases, is not starting up or running correctly.
Full guide for LED flashing after fan05
Orange LED flashing High Engineer
Your boiler has detected an issue with the sensor that monitors the temperature of the water in your radiators, preventing it from heating your home properly.
Full guide for Orange LED flashingon the Sime Format C Combi
Ignition ElectrodeFlame Sensing ProbeGas ValveCirculating PumpWater Flow SwitchLimit 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