BoilerFaultCodesThe 6 most common faults
01
Boiler off High DIY-safe
Your boiler is currently switched off or not receiving power, so it cannot provide heating or hot water.
What to try first
Check that the boiler's power switch or plug is turned on.Check your home's fuse box or circuit breaker to ensure the boiler fuse hasn't tripped.Verify that your room thermostat and timer are turned on and calling for heat.
02
Ignition lock High Engineer
Your boiler has stopped working because it failed to light the flame, and it has locked itself for safety with a red light showing.
Full guide for Ignition lock03
Water flow switch (FL) or limit stat (TL) intervention High Engineer
Your boiler has temporarily stopped working because it has detected a problem with the water flow or an internal safety thermostat has been triggered.
Full guide for Water flow switch (FL) or limit stat (TL) intervention04
Fan fault High Engineer
Your boiler has stopped working because it cannot detect that the internal fan, which safely removes combustion gases, is operating correctly.
Full guide for Fan fault05
Flame detection circuit fault Emergency Engineer
Your boiler is failing to properly detect the burner flame, so it has safely shut down to prevent any unburned gas from building up.
Full guide for Flame detection circuit fault06
100°C safety stat(TS) or smoke stat(TF) intervention Emergency Engineer
Your boiler has automatically shut down because it detected dangerously high temperatures inside the unit.
Full guide for 100°C safety stat(TS) or smoke stat(TF) interventionon the Sime Format HE Combi
FUSEPCBPower SupplyIgnition electrodeGas valveWater 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