BoilerFaultCodesThe 6 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 has been left in its internal testing mode, which is usually only used by engineers during a service or setup.
Why it happens: This fault occurs when the 'Service' dial on the internal control board has been left in the maximum position after a visit from an engineer. It is a manual setting used to force the boiler to run at full power for testing purposes, rather than being an electronics failure. To clear it, the dial simply needs to be turned back to the normal 'off' or '0' position so the boiler can return to its regular operating mode.
Locate the horizontal dial on the front of the boiler control panel.Check if the dial is turned all the way to the right (maximum position).Turn the dial back to your desired temperature setting to exit service mode.
Your 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 is switched on and ready to work, but it isn't responding when you turn up the heating or hot water, usually because an external controller or a valve is failing to tell the boiler to start.
Full guide for Light ONYour boiler has been left in a manual testing mode after a service or inspection and needs to be switched back to its normal operating setting.
Why it happens: This issue occurs when a technician accidentally leaves a physical switch on the internal control board set to 'minimum' after performing a service or safety check. Because the boiler is being told to stay at its lowest power setting for testing purposes, it cannot respond to your thermostat's request for full heat. It is a simple human error where a manual override was not deactivated before the boiler casing was put back on.
Locate the service mode switch (or dial) on the boiler control panel.Ensure the switch is moved from the 'Min' or 'Service' position back to the 'Normal' or 'Off' position.Perform a simple reset by pressing the reset button to clear the flashing light.
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