BoilerFaultCodesThe 6 most common faults
Your boiler has detected a safety-related issue and has locked itself out to prevent further damage, requiring a manual reset to restart.
Why it happens: This fault occurs when the boiler fails to detect a stable flame after several attempts to light. The most common reasons include a lack of gas supply, worn-out ignition parts that can no longer create a spark, or a sensor that is too dirty to 'see' the flame even when it is present.
Locate the reset button on the front control panelPress and hold the reset button for at least one secondRelease the button and wait for the boiler to attempt to restartIf the boiler refires, monitor it closely; if the light returns, call an engineer
Your boiler has detected a general internal fault that requires a system reset or power cycle to clear.
Why it happens: This fault occurs when the boiler's internal computer experiences a communication error or fails to complete its safety checking sequence during startup. The most common reason is an electronic glitch within the main control board or a failure in the ignition process that prevents the burner from firing safely. It effectively means the boiler has reached a 'deadlock' where it requires a manual intervention to confirm the system is safe to try again.
Locate the main power switch for the boiler (usually a fused spur nearby) and turn it off.Wait for approximately 30 seconds.Turn the power back on and check if the flashing light stays off and the boiler fires up.
Your boiler has lost power because it is switched off at the front, there is no electricity reaching the unit, or an internal fuse has blown.
Why it happens: This fault occurs when the boiler's internal computer loses its electrical connection, essentially leaving the unit without the power it needs to operate. The most common reasons are that the main power switch on the front panel has been turned off, a fuse on the circuit board has popped, or there is an external issue with the electrical supply to the house. It can also happen if the control board itself has suffered a component failure and can no longer process electricity.
Check that the on/off dial on the front of the boiler is set to a 'on' positionEnsure the main electrical isolation switch (usually a white switch near the boiler) is turned onCheck your home's consumer unit (fuse box) to see if a circuit breaker has tripped
Your boiler is not receiving a signal from your thermostat or internal timer telling it to turn on.
Why it happens: This fault occurs when there is a break in the communication link between your controls and the boiler's main computer. It typically happens because the mechanical timer has worn out or the batteries in your wall thermostat have died, meaning the boiler never receives the 'on' command. In some cases, the internal electronic board that processes these signals has developed a fault and can no longer recognize when heat is being requested.
Check that your room thermostat is turned up higher than the current room temperature.Replace the batteries in your wireless room thermostat if it has them.Ensure your programmer or timer is set to 'ON' or 'AUTO' and that the clock is showing the correct time.
Your boiler has likely detected that it is too cold to operate safely or that moisture has affected its internal electronics.
Why it happens: This fault occurs when the boiler's internal sensors detect that the air temperature inside the casing has dropped below 5°C, posing a risk of freezing. It can also be triggered if high levels of moisture or dampness have reached the control board, causing the electronics to misread the temperature. The system shuts down as a safety precaution to prevent structural damage from ice or electrical short circuits.
Increase the ambient temperature in the room where the boiler is located to above 5°C.If the area is damp, use a dehumidifier or improve ventilation to dry out the surroundings.Reset the boiler and wait for it to reach a normal operating temperature.
Your boiler has shut down because it has reached a temperature that is too high, often due to a lack of water or a circulation issue.
Why it happens: This fault occurs when the water inside the boiler gets too hot too quickly, triggering a safety switch to prevent damage. The most common reasons are a pump that has stopped moving the water around, a blockage in the pipework, or a sensor that is sending incorrect readings to the control board.
Check the pressure gauge and top up to 1.5 bar if it is lowEnsure all radiator valves are open to allow water flowPress the reset button once the boiler has cooled down
on the Worcester Bosch Greenstar Si
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