BoilerFaultCodesThe 6 most common faults
Your boiler has switched itself off for safety because it has reached an excessively high temperature, likely due to a circulation problem.
Why it happens: This fault occurs when the water inside the boiler gets too hot, causing a safety sensor to trip and shut the system down to prevent damage. The most common reasons are a pump failure that stops water from moving or a blockage within the pipework that restricts flow. In some cases, the sensor itself may have developed a fault and is incorrectly reporting a high temperature.
Check that your radiator valves are fully open and not turned down.Ensure there is enough water pressure in the system by checking the gauge.Try resetting the boiler by pressing the reset button to see if it clears the lockout.
Your boiler has failed to ignite and has shut itself down for safety, meaning you will not have any heating or hot water until it is reset or repaired.
Why it happens: This fault occurs when the boiler's control system attempts to light the burner but fails to detect a flame after several attempts. The most common reasons include a lack of gas reaching the unit, a worn-out component failing to create a spark, or a sensor that can no longer confirm the flame is safely lit.
Check if your gas meter has credit and other gas appliances like the hob are workingPress the Reset button (indicated by a flame with a cross through it) for five secondsEnsure the boiler pressure gauge is between 1.0 and 1.5 barIf it is freezing outside, check that your white plastic condensate pipe hasn't frozen
Your boiler has switched itself off to prevent damage because the internal water temperature has become too hot.
Why it happens: This fault occurs when the water inside the boiler exceeds a safe temperature limit, triggering a safety shutdown. The most common reasons are a failed pump that isn't moving water around the system or a build-up of sludge and debris that has blocked the flow within the narrow pipes of the heat exchanger.
Check that all radiator valves are fully open to allow water to circulateEnsure the boiler pressure is between 1 and 1.5 bar on the digital displayWait for the boiler to cool down and press the 'Reset' button (flame symbol)
Your boiler has detected that the water pressure is too low to operate safely and has shut down to prevent damage.
Why it happens: This fault occurs when the system lacks enough water pressure to circulate heat safely, causing the boiler to shut down to protect the heat exchanger. The most common reason is a slow leak somewhere in your radiators or pipework, or simply that the system needs to be topped up after air was bled from the radiators.
Locate the filling loop (usually a silver flexible hose) underneath the boiler.Open the one or two small valves on the hose until you hear water flowing.Watch the pressure gauge and close the valves when the needle reaches 1.5 bar.
Your boiler has stopped working because the water isn't moving through the system fast enough, causing the internal temperature to rise too quickly.
Why it happens: This fault occurs when the boiler detects that water is heating up too quickly because it isn't being pushed around your radiators fast enough. The most common reasons are a pump that has seized or failed, a blockage caused by sludge in the pipework, or air trapped within the system preventing flow.
Check that all radiator valves are fully openEnsure the boiler pressure is between 1.0 and 1.5 barBleed all radiators to remove trapped air from the system
Your boiler is heating up too quickly because water isn't flowing through the system properly, causing it to shut down to prevent overheating.
Why it happens: This fault occurs when the boiler detects that the internal water temperature is rising much faster than it should, triggering a safety shutdown. The most common cause is a blockage in the system or a failing pump that prevents water from moving away from the heat exchanger quickly enough. It can also be caused by air trapped in the pipework or a buildup of debris known as central heating sludge.
Check that your boiler pressure is between 1.0 and 1.5 bar on the digital display.Ensure all radiator valves and isolation valves under the boiler are fully open.Bleed your radiators to remove any trapped air pockets that might be blocking flow.
on the Glow-worm Ultracom 2 CXI
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