BoilerFaultCodesThe 5 most common faults
01
Red LED on (solid) High DIY-safe
Your boiler has failed to light the gas to create a flame, causing the system to shut down for safety.
What to try first
Check that your gas emergency control valve is open and other gas appliances (like a hob) are working.Press and hold the reset button for a few seconds to see if the boiler restarts.Ensure there is credit on your gas meter if you use a prepay system.
02
Red LED after flow switch High DIY-safe
Your boiler has stopped working because water is not moving through the system properly, which could be caused by a blockage, a faulty pump, or trapped air.
What to try first
Check that your boiler pressure is between 1.0 and 1.5 barEnsure all radiator valves and any isolation valves are fully openTry bleeding your radiators to remove any trapped airReset the boiler using the control knob
03
Red LED on (after safety stat) High Engineer
Your boiler has shut down because it triggered a safety sensor or overheated, meaning it needs to cool down and be inspected.
Full guide for Red LED on (after safety stat)04
LED flashing after fan High Engineer
Your boiler's fan isn't starting or spinning at the correct speed, which prevents the boiler from safely lighting.
Full guide for LED flashing after fan05
Orange LED flashing High Engineer
Your boiler has detected a problem with the temperature sensor that monitors your radiators, meaning it cannot safely gauge how hot the water is.
Full guide for Orange LED flashingon the Sime Format System HE
Ignition electrodeFlame sensing probeGas valvePCBCirculating PumpWater Flow Switch
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