BoilerFaultCodesThe 6 most common faults
Your boiler has lost its electrical supply and won't turn on, meaning it cannot provide any heating or hot water.
Why it happens: This fault occurs when the electrical circuit feeding the boiler is interrupted, meaning the control system has no power to operate. The most common reasons are a blown fuse in the external wall switch, a tripped circuit breaker in your fuse box, or a failure of the internal components that manage electricity within the boiler itself.
Check if other appliances in the house have powerInspect your home's main fuse box (consumer unit) for any tripped switchesCheck the fused spur switch near the boiler to ensure it hasn't been turned off or the fuse hasn't blown
Your boiler has shut down because it is not burning fuel efficiently or safely, which is usually caused by a faulty internal sensor or an incorrect gas mixture.
Why it happens: This fault occurs when the boiler's internal monitoring system detects that the flame is not burning at the correct intensity or stability required for safe operation. The most common reason is that the sensing electrodes have become worn or dirty, preventing the boiler from 'seeing' the flame even if it is lit. It can also be triggered if the gas valve is not delivering the precise amount of fuel needed for clean combustion.
Full guide for BCYour boiler's internal computer has encountered an error and is unable to communicate properly with the rest of the system.
Why it happens: This fault occurs when the boiler's main control board experiences an internal electronic failure or a loss of data. It is often caused by a component failure on the circuit board itself or a loose connection in the internal wiring preventing the computer from sending instructions to the burner. In some cases, an external power surge or moisture ingress can also damage the sensitive electronics inside.
Full guide for 74Your boiler’s internal flame sensor is dirty and can no longer see the fire, causing the system to shut down for safety.
Why it happens: This fault occurs when the photocell, which acts as the boiler's 'eye,' becomes coated in a thin layer of soot or oily film. Because the sensor can no longer detect the light from the flame, the boiler shuts down as a safety precaution to prevent unburnt fuel from building up. This is usually caused by natural wear over time or a slight change in the burner's combustion quality.
Full guide for 75Your boiler is having trouble creating the spark or flame needed to start up because the internal ignition components are out of alignment.
Why it happens: This fault occurs when the spark electrodes inside the boiler's burner move out of their precise position or become worn down over time. Without the correct spacing, the boiler cannot create a strong enough spark to ignite the fuel, or it may fail to detect that a flame has successfully started.
Full guide for 78Wind or air is blowing back down your boiler's chimney, which is preventing the exhaust fumes from escaping safely.
Why it happens: This fault occurs when strong gusts of wind enter the flue pipe and push air back into the boiler, disrupting the internal pressure balance. The most common reason is that the flue terminal is positioned in an area exposed to high winds or is missing a protective guard, causing the safety sensors to trigger a shutdown to prevent fumes from entering the home.
Full guide for 72on the Worcester Bosch Greenstar Utility 18/25
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