BoilerFaultCodesThe 6 most common faults
Your boiler has encountered an internal electronic communication error that is preventing it from running safely.
Why it happens: This fault occurs when the boiler's internal computer brain fails to communicate properly with the other sensors and components inside the unit. It is usually caused by a physical failure of the main circuit board or a loose connection in the internal wiring harness that prevents data from traveling through the system. Because the boiler cannot verify that it is operating safely, it stops running as a protective measure.
Press the 'Reset' button on the control panel once.Wait up to 15 minutes for the boiler to restart.If the code persists, do not attempt further repairs and contact a Gas Safe engineer.
Your boiler has likely experienced a glitch or internal hardware communication error within its main computer system.
Why it happens: This fault occurs when the boiler's main control board experiences a data processing failure or loses connection with internal sensors. It is typically caused by a genuine electronic failure within the computer chip or electrical interference disrupting the signal between components. While a simple reset may temporarily clear it, the code usually indicates that the internal hardware has reached the end of its reliable lifespan.
Locate the main power switch for the boiler and turn it off.Wait for approximately 30 seconds.Turn the power back on and press the 'Reset' button on the boiler fascia for 3 to 5 seconds.
Your boiler has stopped working because it cannot light the burner, which most often happens when your oil tank is empty or there is an issue with the oil supply.
Why it happens: This fault happens because the burner has failed to establish a flame after several attempts, causing the boiler to shut down for safety. The most common cause is simply a lack of fuel reaching the burner, either because the oil tank is empty or there is a blockage in the supply line. It can also be caused by a dirty sensor that can no longer 'see' the flame or a worn-out nozzle that isn't spraying fuel correctly.
Check the level of oil in your fuel tank to ensure it hasn't run dry.Verify that the oil supply valves to the boiler are fully open.Press the reset button on the control panel once to see if the burner restarts.
Your boiler has detected an internal error and is waiting for you to look at the display to reveal the specific fault code causing the problem.
Why it happens: The 'Alert' notification appears when the boiler’s central brain detects that a specific component is no longer operating within its safe or expected limits. This usually happens because an internal part, like a sensor or the fan, has either failed completely or is sending inconsistent data to the control board. It acts as a protective 'blanket' message that stops the boiler to prevent further damage until the underlying technical issue is identified.
Full guide for AlertYour boiler has stopped working because it has been reset too many times while trying to clear a burner fault, and it has now locked itself for safety.
Why it happens: This fault occurs when the boiler's safety system permanently locks itself to prevent damage after the manual reset button has been pressed too many times. It is triggered by an underlying failure in the burner system, such as a blocked oil nozzle or a faulty ignition component, which prevents the flame from lighting correctly. Once this specific code appears, the boiler will no longer respond to simple resets and requires a professional intervention to clear the memory.
Full guide for 9F856Your boiler is temporarily shutting down because it has detected that the internal water temperature has become dangerously high.
Full guide for i10on the Worcester Bosch Greenstar Heatslave II
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