BoilerFaultCodesThe 6 most common faults
Your boiler has lost water pressure, meaning there isn't enough water inside for the pump to circulate properly.
Check the pressure gauge on the front of the boiler; it should be between 1 and 1.5 bar.Locate the filling loop (braided silver hose) underneath the boiler and open the valves to top up the pressure.Bleed your radiators to remove any trapped air, then re-check the pressure gauge.
Your boiler has overheated because the water inside is not moving correctly or has become too hot, causing a safety sensor to shut the system down.
Check the pressure gauge is between 1.0 and 1.5 barBleed all radiators to remove trapped airEnsure at least two radiator valves are fully open to allow water flowReset the boiler using the reset button
Your boiler is failing to light because it cannot detect a flame, meaning your heating and hot water will not work.
Check that your gas meter is turned onIf you have a pre-payment meter, ensure you have creditCheck if other gas appliances, like a cooker, are workingPress the Reset button for five seconds
Your boiler's internal computer brain is unable to recognize a small identifying chip that tells it how to operate safely.
Full guide for 4 flashesYour boiler has identified that the fan responsible for clearing exhaust fumes is not spinning at the correct speed, preventing it from starting safely.
Full guide for 5 flashesYour boiler's temperature sensor is failing to communicate correctly, which prevents the system from knowing how hot the water is and causes it to shut down for safety.
Full guide for 6 flasheson the Worcester Bosch Greenstar 25CDi
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 Worcester Bosch 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