BoilerFaultCodesThe 6 most common faults
01
S.2 High DIY-safe
Your boiler is struggling to circulate water properly around the system, which is preventing it from heating up correctly.
What to try first
Check that your boiler pressure is between 1.0 and 1.5 bar on the gaugeEnsure all radiator valves are fully openTry bleeding your radiators to remove trapped air
02
S.3 High DIY-safe
Your boiler is struggling to light the burner to provide central heating, meaning your radiators will not warm up.
What to try first
Check that your external gas valve is fully openIf you have a prepay gas meter, ensure you have active creditTry resetting the boiler by turning the main control dial to 'reset' and back
03
S.1 High Engineer
Your boiler is unable to start its heating process because the internal fan, which safely clears waste gases, is failing to spin up properly.
Full guide for S.104
CONNECTION High Engineer
Your boiler's main control panel has lost communication with its internal electronic brain, meaning the system cannot receive or process any instructions to work.
Full guide for CONNECTION05
S.7 Medium DIY-safe
Your boiler has finished its heating cycle but is struggling to circulate water to cool itself down correctly, which can cause the system to pause.
What to try first
Ensure all radiator valves (TRVs) are fully open to allow water to circulateCheck that the boiler pressure gauge is between 1 and 1.5 barGently bleed air from the radiators if they feel cold at the top
06
S.6 Medium Engineer
Your boiler is failing to shut down properly after a heating cycle because the internal fan is stuck running for too long.
Full guide for S.6on the Vaillant Combi Compact 242EB
Circulating pumpFlow pipe sensorIgnition ElectrodeGas ValvePrinted Circuit Board (PCB)Fan Assembly
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 Vaillant 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