BoilerFaultCodesThe 6 most common faults
Your boiler has detected that there is not enough water pressure in the system to operate safely, which has caused it to shut down.
Why it happens: This fault occurs when the water pressure inside your heating system drops below the minimum level required for the boiler to heat up safely. The most common reason is a slow leak from a radiator valve or pipework, though it can also happen if you have recently bled your radiators and didn't top the pressure back up.
Locate the external filling loop, which is usually a flexible silver hose with two small valves underneath or near the boiler.Slowly open both valves to allow water into the system while watching the pressure gauge.Close both valves tightly once the pressure gauge reaches between 1.0 and 1.5 bar.
Your boiler has stopped working because it cannot light the gas to start the heating process.
Why it happens: This fault occurs when the boiler attempts to light the gas but fails to detect a flame after several tries. It is often caused by a worn-out ignition lead that can no longer create a spark or a faulty gas valve that isn't letting fuel into the burner. In some cases, a blocked flue or a problem with the internal electronics can also prevent the ignition process from completing.
Check that your gas emergency control valve is in the 'on' positionIf you have a pre-pay gas meter, check that you have sufficient creditPress the reset button (indicated by a flame symbol with a cross through it) for five seconds
Your boiler has stopped working because a sensor that monitors the temperature of the water leaving the unit has failed or disconnected.
Why it happens: This fault occurs when the boiler's internal computer loses communication with the sensor that monitors the temperature of the water sent to your radiators. The most common reason is that the sensor has physically failed due to age or internal wear, though it can also be caused by a loose wire or moisture damaging the electrical connection.
Full guide for F10Your boiler has lost communication with the temperature sensor on your hot water cylinder, meaning it cannot tell how hot your water is.
Why it happens: This fault occurs when the boiler can no longer receive an electrical signal from the temperature probe attached to your hot water tank. The most common cause is a failure of the sensor's internal components or a loose connection in the wiring that links the cylinder to the boiler's main control board.
Full guide for F13Your boiler has switched itself off because the waste gases are getting too hot, which is usually caused by a lack of water circulating through the system.
Why it happens: This fault occurs when the internal sensors detect that the exhaust gases are getting dangerously hot, triggering a safety shutdown to prevent damage. The most common reason is that heat isn't being carried away from the burner quickly enough, usually because the water isn't circulating properly or there is a blockage within the boiler's pipework.
Full guide for F25Your boiler's main control unit is confused because a specific internal configuration component is missing or has the wrong electrical profile.
Why it happens: This fault occurs when the boiler's main control board loses its identity because a small electronic component called a coding chip is loose, damaged, or missing. The chip tells the computer exactly which model of boiler it is controlling, and without this information, the system shuts down for safety. The most common reason is a component failure within the electrical circuit or a loose connection following a recent repair.
Full guide for F43on the Glow-worm Flexicom sx
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 Glow-worm 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