BoilerFaultCodesThe 4 most common faults
Your boiler appears to have no power and the display is completely dead, meaning it cannot provide any heating or hot water.
Why it happens: This fault occurs when the electrical circuit inside the boiler is interrupted, preventing power from reaching the internal components. The most common reason is a blown internal fuse or a failure within the transformer, which acts as the heart of the boiler's electrical system. In some cases, a power surge or a short circuit on the main control board can cause the unit to shut down completely to prevent further damage.
Check if the main electricity switch for the boiler is turned onCheck your home's consumer unit (fuse box) to see if a circuit breaker has trippedReplace the fuse in the external boiler spur switch with a new 3-amp fuse
Your boiler has detected a critical internal fault and has locked itself out to prevent damage or unsafe operation.
Why it happens: This fault occurs when the boiler's internal computer detects a conflict or failure in its safety sensors or the fan that clears exhaust gases. The most common reason is that a sensor has stopped reporting the correct temperature or the fan is no longer spinning at the required speed, causing the system to shut down for safety. Since the boiler cannot confirm the system is venting or cooling correctly, it locks itself out to prevent overheating.
Full guide for Fast flashYour boiler is trying to start but the flame isn't catching, so it has safely shut itself down to prevent gas from building up.
Full guide for flame lock-outYour boiler has detected that it cannot safely get rid of exhaust gases, so it has stopped working to prevent a buildup of dangerous fumes.
Full guide for Slow flash (unspecified rate)on the Worcester Bosch Greenstar 12Ri
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