BoilerFaultCodesThe 6 most common faults
Your boiler has sensed that the water inside is getting too hot and has shut itself down to prevent damage.
Why it happens: This fault occurs when the internal water temperature exceeds safe limits, forcing the boiler to shut down for safety. The most common reasons are a failed pump that isn't moving water around the system or a blockage caused by a buildup of sludge and debris in the main heat exchanger.
Check that your radiator valves are fully open to allow water to circulate.Ensure your boiler pressure is between 1 and 1.5 bar on the front gauge.Press the reset button to see if the fault clears after the system cools down.
Your boiler has tried to light several times but cannot detect a flame, meaning it has locked itself out for safety.
Why it happens: This fault occurs when the boiler's control system fails to detect a flame after several ignition attempts. The most common reasons include a lack of gas reaching the burner, worn-out ignition components that can't create a spark, or a sensor that is too dirty to 'see' the flame even if it has lit.
Check if other gas appliances, like a hob, are working to ensure gas is reaching the propertyCheck if your gas meter credit has run out or if the semi-solid valve is bumpedIf it is freezing outside, check if your white plastic condensate pipe is frozen and defrost it with warm waterPress and hold the 'Reset' button for five seconds
Your boiler's internal components are having trouble communicating with the main control board, preventing the system from starting up.
Why it happens: This fault occurs when the boiler's main control board loses its digital connection with internal components like the display or the heat engine sensors. It is usually caused by a loose connection in the internal wiring or a physical failure of the electronic circuit board itself, preventing the boiler from sending start-up commands. In some cases, moisture ingress or a power surge may have damaged the sensitive communication pins on the board.
Full guide for A8Your boiler's internal computer brain cannot find its identity chip, which prevents it from operating safely.
Why it happens: This fault occurs when the boiler's main control board loses communication with a small, specialized chip called a coding plug that contains the unit's unique settings. The most common reason is that the plug has vibrated loose or become faulty, but it can also happen if moisture or a power surge has damaged the circuitry on the main control board.
Full guide for B1Your boiler's fan is spinning faster than it should, which is preventing the system from safely managing the air and gas flow needed for heating.
Why it happens: This fault occurs when the boiler's internal computer loses the ability to control how fast the fan is spinning, causing it to run at an unsafe maximum speed. The most common reason is a communication failure between the fan motor and the main control board, often caused by a worn-out component or a loose electrical connection. Because the boiler cannot accurately balance the air and gas mixture, it shuts down as a safety precaution.
Full guide for C6Your boiler thinks a flame is still burning even after it has been told to turn off, which is a safety conflict that stops the system from running.
Why it happens: This fault occurs when the boiler's internal sensors continue to detect a flame or electrical current even after the gas valve has been closed. The most common cause is a carbon-clogged sensing electrode sending a false signal, but it can also be triggered by a fault in the main control board or a short in the internal wiring. Because it is a safety concern for the boiler to 'see' fire when it shouldn't, the system shuts down completely to prevent damage.
Full guide for F7on the Worcester Bosch Greenstar i
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