BoilerFaultCodesThe 5 most common faults
01
Red LED on (solid) High DIY-safe
Your boiler has stopped working because it couldn't light the flame, which has triggered a safety lockout to prevent issues.
What to try first
Check if your gas supply is turned on (check other gas appliances or credit on your meter)Press and hold the 'R' (reset) button for a few seconds to restart the systemCheck if your condensate pipe has frozen if it is currently very cold outside
02
Red LED after flow switch High DIY-safe
Your boiler has detected that water is not circulating correctly through the system, which has likely caused it to shut down to prevent overheating.
What to try first
Check that your boiler pressure is between 1.0 and 1.5 bar on the gaugeEnsure all radiator valves and isolation valves under the boiler are fully openTry bleeding your radiators to remove any trapped air pockets
03
Red LED on (after safety stat) High Engineer
Your boiler has switched itself off for safety because it has detected an internal temperature that is too high.
Full guide for Red LED on (after safety stat)04
LED flashing after fan High Engineer
Your boiler has detected that the internal fan, which safely clears exhaust gases, is not spinning or communicating correctly with the system.
Full guide for LED flashing after fan05
Orange LED flashing High Engineer
Your boiler has detected a problem with the internal temperature sensor that controls your central heating, meaning it cannot safely monitor how hot the water is getting.
Full guide for Orange LED flashingon the Sime Format HE Combi
Ignition electrodeGas valvePrinted Circuit Board (PCB)Circulating 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