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 stopped working because the fan cannot properly push exhaust fumes out or pull fresh air in through the flue pipe.
Why it happens: This fault occurs when the boiler's internal safety sensors detect that the fan is not moving air through the system at the required speed. The most common reasons are a physical blockage in the flue pipe outside, a build-up of debris within the fan itself, or a failure in the electronic sensor that monitors air pressure.
Check the external flue pipe for obvious visual obstructions like bird nests, leaves, or debrisEnsure the external flue terminal is not covered by garden furniture or building materialsGently clear any visible snow or ice buildup from the flue outlet
Your boiler has become too hot and has automatically shut itself down to prevent damage or safety issues.
Why it happens: This fault occurs when the water inside the boiler reaches an unsafe temperature, triggering a safety switch to cut the power. The most common reasons include a failing pump that isn't moving water away quickly enough or a buildup of limescale and sludge that restricts flow through the heat exchanger.
Wait at least 30 minutes for the boiler to cool down completely.Locate the reset button (usually behind the small circular flap or on the main control panel).Press and hold the reset button for five seconds to see if the boiler restarts.Check that your radiator valves are open and your system pressure is between 1 and 1.5 bar.
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 pump or motor is not receiving power or has stopped working, meaning it cannot circulate heat through your system.
Why it happens: This fault occurs when the boiler's internal pump fails to move water, which prevents heat from leaving the unit and reaching your radiators. The most common reasons are a physical blockage or mechanical failure within the pump motor itself, or a communication issue where the main control board is unable to send electricity to the pump.
Full guide for 73Your 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 74on 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