BoilerFaultCodesThe 6 most common faults
Your boiler has reached an unsafe temperature and has automatically shut down to prevent damage or safety issues.
Why it happens: This fault occurs when the water inside the boiler gets too hot, triggering a safety sensor to cut the power. The most common reasons are a pump failure that stops water from moving or a blockage in the system that prevents heat from escaping the boiler's internal components.
Check that your radiator valves are open and not turned to zero.Ensure there is enough water pressure in the system (typically 1.0 to 1.5 bar).Press the reset button (the flame icon with a line through it) to see if the boiler restarts.
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 sensed that water is not circulating through the system properly, causing a large temperature gap between the pipes leaving and returning to the unit.
Why it happens: This fault occurs when the water inside the boiler heats up too quickly because it isn't moving away into your radiators fast enough. The most common reasons are a failed internal pump that has stopped spinning or a blockage in the pipework that is restricting the flow of water. Either way, the boiler shuts down as a safety precaution to prevent the heat exchanger from overheating.
Ensure all radiator valves and lockshields are fully open.Check that the boiler pressure is between 1.0 and 1.5 bar on the gauge.Bleed all radiators to remove any trapped air pockets that may be blocking flow.
Your boiler is overheating because water isn't moving through the system fast enough, causing it to shut down to prevent damage.
Why it happens: This fault occurs when the water inside the boiler heats up far too quickly, triggering a safety sensor to prevent the unit from melting or cracking. The most common reason is a blockage or a failed pump preventing the water from flowing out of the boiler and into your radiators. It can also be caused by a build-up of sludge or debris in the system that acts like a dam, trapping heat inside the heat exchanger.
Check that all radiator valves are fully openEnsure your central heating bypass valve (if visible) is openTry resetting the boiler to see if the fault clears
Your boiler has failed to ignite and is not receiving a steady gas supply or spark, meaning you currently have no heating or hot water.
Why it happens: This fault occurs when the boiler tries to light but fails three times in a row, leading to a safety shutdown. It is physically caused by a lack of gas reaching the burner or a failure in the ignition system, such as a worn-out spark electrode or a faulty gas valve. If you have a prepay meter, check that you have credit, as a lack of fuel is a frequent trigger for this error.
Check if your gas meter has credit (if on prepay)Ensure your gas stopcock and other gas appliances, like a cooker, are workingPress the Reset button (indicated by a flame icon with a cross) for five seconds
Your boiler was running but the flame has unexpectedly gone out, causing the system to stop working as a safety precaution.
Why it happens: This fault occurs when the boiler successfully lights but cannot maintain a steady flame, causing the system to shut down for safety. The most common reasons include a blockage in the condensate drainage pipe, an unstable gas supply, or a buildup of carbon on the electrodes that monitor the flame.
Check that your gas emergency control valve is open and you have credit on your gas meter.Check if other gas appliances in your home, such as a hob, are working correctly.Press the reset button (indicated by a flame with a line through it) for five seconds.
on 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