BoilerFaultCodesThe 6 most common faults
Your boiler has stopped working because the water pressure is too low for the system to run safely.
Why it happens: This fault occurs when the water level inside your central heating system drops too low for the boiler to operate safely. The most common reasons are a slow leak from a radiator valve or pipework, or the air being manually bled from radiators without the pressure being topped back up.
Locate the filling loop (usually a silver flexible hose) underneath the boiler.Slowly open the one or two taps/levers on the filling loop to allow water into the system.Watch the pressure gauge until it reaches between 1.0 and 1.5 bar, then close the taps fully.
Your boiler is failing to light because it isn't receiving a gas supply or cannot detect a flame.
Why it happens: This fault occurs when the boiler's internal computer tries to start the burner but cannot establish a sustained flame. The most common reasons are a temporary interruption in your gas supply, a worn-out ignition probe that cannot create a spark, or a faulty gas valve failing to release fuel into the combustion chamber.
Check that your gas emergency control valve (usually by the meter) is in the 'on' position.If you have a pre-payment meter, check that you have sufficient credit.Check if other gas appliances in the home, such as a hob, are working correctly.Press and hold the reset button for five seconds to see if the boiler restarts.
Your boiler has lost its water pressure, which means it has stopped running to protect the internal components from damage.
Why it happens: This fault occurs when the water level inside your central heating system drops too low for the boiler to operate safely. It is usually caused by a small leak in the pipework or radiators, or because air was recently bled from the system without being topped back up.
Locate the filling loop (usually a silver flexible hose) under the boiler.Slowly open the one or two small taps on the hose until you hear water flowing.Watch the pressure gauge until it reaches between 1.0 and 1.5 bar, then close the taps tightly.
Your boiler is failing to light the flame needed to heat your water and radiators.
Why it happens: This fault occurs when the boiler attempts to start up but cannot confirm that a flame has been successfully lit. The most common reasons are that the spark isn't reaching the burner, the sensor that detects the heat is dirty or worn out, or the gas valve isn't opening correctly to provide fuel.
Check that your gas pre-payment meter hasn't run out of creditEnsure other gas appliances in your home, like a hob, are receiving gasPress the 'Reset' button on the boiler control panel to see if it restarts
Your boiler has got too hot and has shut itself down as a safety precaution to prevent damage.
Why it happens: This fault occurs when the internal temperature of the boiler rises above a safe limit, causing it to trip a safety switch. The most common reasons are a failed pump that isn't moving water around the system quickly enough, or a blockage caused by sludge and debris inside the heat exchanger. It can also be caused by an air lock in the pipework that prevents the water from absorbing the burner's heat.
Check that all radiator valves are fully open (especially TRVs)Ensure there is enough water pressure in the system (typically 1.0 to 1.5 bar)Press the reset button once the boiler has had time to cool down
Your boiler has detected an issue with how it monitors water levels, meaning it cannot confirm there is enough water to safely provide heating or hot water.
Why it happens: This fault occurs when the boiler's internal computer loses communication with the sensor that monitors water levels. The most common reason is that the sensor itself has failed due to age or internal corrosion, or the electrical wiring connecting it to the main control board has developed a loose connection.
Full guide for F9on the Glow-worm Xtrafast 96 120
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
Avoid the next call-out bill
Monthly cover means no unexpected bills when your boiler breaks down.
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